Dishonesty

My girlfriend and I have been dating for the past 2 years. For most of the relationship we lived in the same place, but the past 4months we lived states apart. She has a past, as do I. We have both attempted to put them behind us, but for the second time, the past became somewhat of the present.

My girlfriend's ex-fling currently has his own girlfriend, but he still is infatuated with my girlfriend. So every now and again he hits her up on AIM(AOL). Usually when he does she'll tell me because we dont want to keep secrets about that stuff, but this time she didnt. In the back of my mind something wasn't right so last night I went on her computer and looked at the history. Apparently, they've had a few conversations that lasted for hours(up until 1am), sometimes while she was talking to me on the phone or AIM. That part bothered me a little bit because she didnt tell me, but the part that bothered me the most was that the conversation became sexual and she made no efforts to stop it or not continue. They were talking about the things they use to do and how much they missed it(they were very vivid).

This is the second time something like this has happened, each time with different ex's. However, the last ex it happened with she has not had contact with for most of the year, and this is the only ex that is really left from her past. I'm not really sure how or what to feel, but I love her very much and there's no other person that I've been with that treats me as well as she has/does. I feel bad for going through her stuff too. Am I in the wrong?