Luke:
You have latin roots so Luke decided to call you mi amor. He was really bad with the whole accent thing in the begining but now he got really good, well at least with this one word.
Ashton:
He likes to call you baby girl. He said he wouldn't stop until you really gave him his baby girl, his daughter, and then he would call you his angel because he think thats the only other suitable nickname since 'you saved him so many times' as he would say.
Calum:
When you and Cal didn't even started dating he would always call you Boo for no special reason, so when you became official he just kept that nickname.
Michael:
Michael calls you cutie, just because he likes the sound of it, and the smile you give him after that. Also, that's a word you used to describe him first time that you met, so it's kind of special to him.