Is this cheating? What do you think?

My husband has always been a good husband. His job has become stressful over the past year or so, and he works a lot in the evenings. He doesn't speak with me about his feelings, but is caring and friendly. We have just been on holiday, and he could not use his blackberry as we had no reception. I only took my computer, so he checked his emails etc on there. When we got home I signed on to check mine, and his facebook was logged in to the inbox. I thought it was mine and clicked on a message wondering who the hell it was from. ANyway, over the past year, my husband has been having in depth conversations with a girl he used to 'hook up' with. They are inappropriate - from him desperately pleading with her to come on to chat, to discussing their naked bodies, to our relationship. I confronted him and he denied it was sinister, just joking, yet he does not have 'girl friends' and used language/things I would never have thought he would use apart from with me. I know that I wouldn't. I have no idea what to think. We have 3 children, thus I really want to believe I am making the right decision. We have always been very upfront about thoughts on cheating and what would happen, and now I cant seem to think straight. Please give me your opinion! Thanks