Think my girlfriends cheating

So a couple of months ago my girlfriend and I were having a few problems (money issues mostly) which we had talked about and we're slowly resolving. Even though we were sorting things out I could tell something wasn't quite right and I found out she was messaging a guy on Facebook. I have no problem what so ever with her talking to other people so I didn't think anything of it and didn't confront her about it. As time went on she was messaging him more and more to the point where it felt like she completely forgot about me and would spend hours just sat talking to him. I mentioned several times that it was bothering me and she just told me nothing was going on and not to worry. One weekend she went to stay at a friends which I thought would do her good to get away for a while and let her hair down, or a t least it though. I found out that she got quite drunk on Saturday night and was txting this guy until 4 in the morning, flirting and asking each other questions that I found extremely inappropriate. So I confront here about it and she said it didn't mean anything and it was just joking around, but the next day the same thing happened. When she got home we had a proper chat about everything that had happened, I asked if she still wanted to be with me and she assured me that she did and it would never happen again and that it was a mistake. I gave her the benefit of the doubt and forgave here and things got better between us and I thought no more about it , water under the bridge if you like. Now yesterday I found out that she has started talking to him again and I honestly don't know what to do. Again she has assured me that nothing like before will happen again "we are just friends nothing more" where her words. As I said before I have no problem with her talking to other people at all but if it's turns into happened before I don't think I can forgive her. Am I just over reacting here or do you think there is cause for concern as I love the girl, but if I feel like I don't trust her there can't be any future for us.

In all honesty, nothing good will come of her messaging him, if she's hiding the messages from you then yes it is cause for concern. I speak through experience here. My (now ex) husband was messaging a girl he worked with then just before Xmas he decided to up & leave. No explanation, nothing, just gone. Since found out 11 days later he's moved in with her. We were together 13 years. So not a good thing, especially if messages before were inappropriate.