There are so many ways to know what others are up to. Are they happier now that you're not around? It can get even more fucked up (for example) if you find out that they were out on a date night at a vacation house with someone else. And these two are clearing flirting on instagram posts. The number of times I've gone back on two instagram profiles to see them at similar places, raging with bitterness with every tap of viewing stories. In my experience, it's better to confront right away so that you can exit gracefully. And definitely use the "mute" function once you've exited the relationship; ignorance is bliss! Take your time to normalize :)
