What's going on here?

I guess this is just kind of to get some more perspectives here. So, I've always had suspicions of my girlfriend cheating on me. More than other men, maybe. I'll admit to having done some snooping. It's the only thing you can do when they don't talk. If you've read my other questions you'd understand exactly what I've been dealing with up until now, and maybe be a little sympathetic. Anyways, back to the question. I've recently discovered that my current girlfriend and her ex boyfriend have gotten back into contact. It wasn't much of a contact, but regardless she replied to him despite everything I have come to believe up until this point. Now, this was half a month ago; so I'm sure they are either talking by other means, or not talking at all. This initial first little encounter, it doesn't seem like much; but if you consider what has happened in the past, then it is to worry about. The last time they had contact was when her and my ex best friend were dating. I think it was about half a year- 3/4 of a year through, she got back in contact with him. Totally played off the 8 months of being with my best friend as nothing. "Oh, I just found out that he's such a loser, I was going to dump him soon anyways. Would you date me if I did?" Yeah, okay, it could be rationalized, but even still. To go behind the back of somebody you said you loved and talk of leaving them so easily? The feelings don't seem to reciprocate, but seeming as how he has now contacted her, it seems he wants to play again. What should I think of this? Should I be worried? If you ask me if I can trust her, I could never say yes. I love her, but I mean... I've always known she was hiding alot from me. I can see it in her actions now. She acts cold sometimes, and gives me these looks, or even just looks away awkwardly. She's really been quiet and doesn't text me until late in the afternoon if even. As well as I've noticed that the days she claims to be to tired to stay up, her statuses and log in times prove that she has been up hours afterwards. I just don't know what I should think.