What would you do?

If your boyfriend had been a little distant towards you, but you notice a snap chat user that you've never heard of before, and when you ask he gets upset and says you don't have trust whatsoever. What would you think? I know I shouldn't overthink it, but why does she send him snap chats in the first place? And she's on top of the people that he snap chats when I use to be number one. Not only that he called for a break because things were getting "stressful". Would any of you girls make any assumptions?I would like to know what you guys think, or any advice? I mean I'm not jealous but could he possibly developed feelings for her? For gods sake who knows what she could be sending him.