I agree...if it's making you feel bad, then it isn't right, online or in "real life." I also agree that you MUST confront him immediately...you will probably be able to tell from his reaction if it was intentional cheating or simply a few comments taken out of context. Don't forget that typed words are VERY subjective and can easily be taken the wrong way when not combined with body language, etc. as Donald said. Sorry, I've been there before too... and I felt my stomach drop for you too. (same old feeling I guess)

Tip: when it happened to me, I confronted the boyfriend and he denied it. To prove it (one way or the other) to myself, I chatted with him under another screen name/profile to see how far he was willing to go and that's when I realized the truth...