Search Results: getting mad at boyfriend

I've been with my boyfriend for close to two years.
we've broken up once during our relationship, but of course we ended up missing eachother and getting back together. The thing though is, I found out through his ex girlfriend that he slept with her when we were apart. I was mad about that for a while but I forgave him. I also learned that during the time we were apart he hung out with a different girl, one he knew in high school a couple of times. when we got back together, I read messages between them and she was being pretty flirty. no, he didn't really respond back in a flirtatious way,

but in one of the messages she asked him if he wanted to have a sleepover, and he didn't reply with a straight yes or no. then she asked him again if he wanted to have a "pajama party" and he said "yess." When I confronted him about seeing the texts, he said it was all a joke and that he never had a thing with her. I feel paranoid though that he did, even though its been a few years. He couldn't tell me about sleeping with the ex (Shen did) so why would he tell me about the girl if he had slept with her? I know this probably doesn't matter but I have this weird feeling and when I stupidly went to talk to the girl about it, she told my boyfriend that I was crazy. Should I believe him?