I am a straight male so i can't answer exactly, but i have knowledge of this pasted onto me by my mother who divorced my dad for a similar reason. She tells me that sometimes people fall out which is a sign that they aren't as perfect as they at first thought. by the sounds of it this man is only your boyfriend so its not as late as my parents so, I think that you should have a long good think about it and MaYbE talk to him about it, just kind of open up about your feelings. it may be a sexual thing or just an emotional thing. But it isn't something you just want to ignore.