Fëanor
When you start to menstruate and Fëanor finds out, the ellon is completely unsurprised. He spent copious amounts of time learning about the race of Men, including menstruating humans. This includes all the remedies and tips and tricks and you'll essentially be explained how to handle your own period. Even if you've been dealing with it personally for years and Fëanor has not.
He doesn't mean it as a "I'm right, you're wrong.", Fëanor is just genuinely trying to be helpful and his fascinated by the phenomenon. (Because elven women probably don't menstruate, or not in the same way we do.) So be prepared for any and all questions he may have about your personal experience. Though he will do this completely in private, he's got enough wits to not flat out embarrass you.
Even despite all his questions and suggestions about your period, Fëanor is actually helpful through the ordeal. He works on trying to make menstrual products better, as well as fine tuning any remedies he's learned from the other human women and noting what works for you and what doesn't. So by the end of your first few periods around him, he has an extremely comprehensive care list he runs off of to make sure your period runs smoothly.
Anticipate for any and all affectionate needs to be met no matter what it is. Sex? Cuddles? Kisses? You've got it. Though you two may butt heads pretty heavily when it comes to the mood swings, as sympathetic as he tries to be about it, sometimes he can't keep a hold of his own emotions either. But Fëanor still tries to be considerate, knowing that you can't help it.
Maedhros
When you first start menstruating and have an accident either bleeding through your clothes or onto the sheets, Maedhros grows extremely concerned and internally panics. You are bleeding from some place he's not known women to bleed from before. And with all the trauma he has, Maedhros is concerned if something transpired to you while he was sleeping.
You'll naturally have to explain that, no nothing traumatic happened to you and that yes this a common and natural occurrence for you. Maedhros is going to be very hesitant about taking your words at face value. In the event he's afraid something did happen and you are lying to him. Of course he doesn't show this outwardly, but that doesn't mean it isn't there.
Once Maedhros is comfortable with the fact that this is in fact normal and you are as okay as you can be, he relaxes considerably. Getting you the things you need should you need him to do it. Maedhros tries to be warmer during your menstrual cycle, trying to be more affectionate and patient and less disconnected from the world. Though there's not always a guarantee he will be, he still tries his best to meet your emotional needs.
Maglor
With Maglor when your menstruation happens, he's very patient and gentle in asking you if everything is alright. At your response about how you are menstruating, Maglor asks what it is you need for him to do to help you out. He will happily go pick up anything you need him too- pain medicine, salves, any products, food, new sheets or clothes. He's happy to help.
Maglor doesn't press you on a lot of questions, even though he doesn't full understand what your menstrual cycle is. Just accepting that you do this once a month (maybe more or less) and that it's apart of a normal life for you. If you do divulge any information about your cycle to him, Maglor just nods along listening diligently.
Maglor is extra affectionate and considerate of anything that you need from him. Whether it be time to cuddle in your shared chambers for several hours or you just need to be away from him. Maglor is happy to give it to you. Expect for him in your time cuddling, to get soft and sweet songs to help lull you into sleep or keep you nice and relaxed.
Celegorm
Celegorm is not the most sensitive, and not always the most understanding elf around, and that doesn't change when you start menstruating around him. It doesn't help that his genuine curiosity can help make him sound more insensitive as he asks you questions about it. They really just sound like he's being a critic, when in actuality he just doesn't understand.
"Can't you control it?" or "Why were you not better prepared for this?" He genuinely means no harm by it and is utterly confused when you are upset by his questions. He is even more surprised when you explain to him that no, you can't control it- unfortunately and that menstruation can be unpredictable. He is even more surprised when something like cramps prevents you from being able to do things.
Celegorm is honestly just tries to make light of the situation and instead focuses on picking at you a little bit. Calling you cute when you are all frustrated and moody, and laughing some if you ask him to rub your stomach because of the cramps. Celegorm does try to be helpful, but requires instruction for quite a while on what it is you need or want out of him when on your cycle.
Caranthir
Caranthir knows what menstruation is, and while it is a topic that generally makes him uncomfortable, he never makes you feel uncomfortable just because he is. Even still despite that discomfort Caranthir works hard to make sure that you are comfortable, even if when you talk about it he is blushing deeply the whole time. He does eventually grow out of that, it's just the first several times he's extra blushy and uncomfortable. But he listens to you diligently.
Caranthir makes sure that any and all of your products come from other menstruating humans. While he does take the advice of his healers, he has more trust in menstruating humans than he does his healers. Who else would know better than them? In the event that the other humans are not available, he has his healers learn things from them, so it's the same advice and things that you would get from your fellow Men.
He keeps everything running for you as smooth as he possibly can, and as discreet as you'd like for it to be. Caranthir also makes sure the Healers are aware that it is a private matter. Caranthir is also extra sweet on you during this time, happy to rub your stomach or back, giving you extra forehead kisses and embraces just wanting to make you feel better.
Curufin
With Curufin, like his Atar, he would've spent copious amounts of time studying the race of Men. So when he met you and started courting you, he was completely unbothered when you started menstruating. If anything, he picked up on it before you ever openly talked about it with him, and spent time learning what it is you did and what medicines you took.
So when you finally broach the subject with him, he simply tells you, "I know." very calmly, and he has a few things to add about what you've been doing. That is wrong to him - it doesn't help he's actually right. Curufin works everything down to an absolute science with your period.
While he won't explain to you how your period works, he is quick to call out things- in private- about what he knows doesn't work and does work with you. He always makes sure that your room is stocked with the things you need, herbs, medicines, menstrual products and the like. Curufin also like clockwork knows when you need affection, having picked up on that too and sets aside time daily for such things.
Amrod
Amrod has knowledge in Healing, so when you start menstruating, his immediate reaction is to try and make it stop and ask why you are bleeding. (Lucky you this happens on a day where he isn't selectively mute). So you will have to stop him from pulling out everything under the sun to tend to you "wound".
Though when you explain to Amrod that it's normal and natural for humans- not all of course- to menstruate, Amrod then asks what it is you need. Because clearly that must be uncomfortable and you are... leaking. So when you tell him, Amrod makes everything that you need from tonics to salves to provide you with any and all relief that you can get.
As for your menstrual products, Amrod will learn how to make those as well as they are a necessity and a medical related need. As for the emotional or physical needs from, Amrod is happy to let you hang all over him. Not being a big day time cuddler or very clingy, but at night he's happy to give you all the cuddles you need.
Amras
With Amras when you start menstruating, he is very go with the flow. Oh so you've had an accident in the bed? No big deal, he's happy to gather up all the sheets and clothes to get ready to wash it. You are cramping really bad and don't think you can help him with it? Don't worry about a thing.
Amras will only divulge to his brother that you are menstruating and that you are cramping, asking if he has anything that can help ease the discomfort and pain. So expect that you will get top quality remedies as Amrod takes it extremely seriously.
Amras is extra clingy to you during this time, wanting to make sure that you are alright and comfortable. Happy to assist you with whatever you need, especially when you are in pain and having days where your cramps are so bad you can't move. He will even neglect his own chores and duties to be with you- Amrod is fortunately very understanding of this and picks up some of the slack in the house for you both.
Celebrimbor
Celebrimbor has been around long enough to run into all walks of life, menstruating humans and all. So when you start to menstruate, he knew it would happen eventually and is very patient with you too. He doesn't ask questions, he just helps and comforts you.
He has a few elves help him discreetly clean up the mess that may have been made, and sends for a healer to bring you any pain relievers you need. Celebrimbor is very smooth and casual about the whole thing and even tells you to take the time off you need from any of your duties. Unfortunately, he can't with all of his own duties.
Though Celebrimbor checks in on you regularly and never parts without a kiss or truly making sure you are okay. Celebrimbor will do what he can to finish with his day as early as possible so he can join you as soon as possible. He does what he can to make sure that you are comfortable and feeling as loved and happy as possible during such an uncomfortable time.