My boyfriend has been acting very different and sneaky Lately. And he left his facebook up on my computer?

I opened my laptop clicked facebook and clicked the messages only to realize they werent my messages. I noticed one from my boyfriends ex boyfriend who lives in california (we live in NY) She said i like your new pics up from your brothers wedding, you look handsome. And he said thank you! Your beautiful! Then she said your gf looks very pretty too. He replied saying, you're prettier you sexy devil ;) By the way were 20 and 21. Now, i dont know what to do about this. I feel very hurt because he has looked me in the eyes and have said over and over how he could never cheat on me or hurt me. Isnt that cheating? Isnt that hurting me? What should i say to him? As soon as i say i know you did **** behind my back. He's just going to say no i didnt. If i bring up i saw his fb he could get mad and start yelling and get pissed off and call me a stalker or something.