Has more happened?..... What do I do now?

My girlfriend was really close to this guy at work and he was txting her constantly. But she said they were only friends... I accepted this and said no more about it..... They reguarly went out clubbing together because the place they work always arranges nights out for them etc. I read her phone one evening (I know this is a bad thing to do but I had to know what was going on).. and I saw txt messages from him saying how much he liked her and how much he was missing her etc. I didn't mention anything to her that evening... but the next day I called her and asked her ' I know things aren't what they seem with you and him, I know things have happened, so tell me the truth.' and because she had realised that I knew something she admitted that she had kissed him but she said it was when me and her were only going out for a month or so, at the start of our relationship. But now looking back there has been many times that she has gone back to parties with him and stayed over at these parties and been with him all night. We had a massive row about this but I have stayed with her because she says nothing else happened and I don't really have the proof that anything else has happened... I said I couldn't stay with her as long as she is friends with him after this... and she has promised she would never see him again other than in work and as far as I know I think she has stuck by that... Its now been a couple of months since that happened and it is still in my head.. I can forgive her for a kiss but it is that lingering feeling that something else has happened. Any advice? Do you think more has happened? Am I stupid for staying with her?
Should I bring this up again? Its been a while since this happened and I haven't mentioned it, but its still in my head.