Don't know?

Okay so I need opinions!
​Today, while I was with my boyfriend someone he used to be friends with (a girl) messaged me saying "you're dating him right? He's cheated on you" but when I asked what she meant and how.. She just read it and didn't reply. I also showed my boyfriend and we went into a different room to talk about it. He started to look mad and looked like he was about to cry. When I asked why he was mad he said its because I'm accusing him of cheating. I don't know if I should believe he didn't or not. Like its kind of weird why someone would say it if it wasn't true? I'm 16 and he's 17 btw. I don't even know her so I don't think it would be out of some hate towards me..