Is this a sign of cheating...?

My boyfriend and I have been going out for eight months and a few days ago we had a heated argument about how I don't trust him and how I get jealous easily. Well, the reason I'm like this is because when he told me about his past, he cheated on a lot of his girlfriends. He was a true player and he admitted that to me. Although I didn't choose to punish him over his past actions, I still had my eye on this retired player of mine. Well, just now as I was looking on his Facebook page, he wrote on this girl's wall, calling her beautiful. She responded with a simple "What's up?" He seemed to be the one showing the affection, putting little hearts beside his comments and stuff but I really didn't trip because her profile said she was in a relationship. But, my suspicions still kicked. I just wanna know if this is a sign of cheating and what should I do about this situation. Please and thank you! :)