Boyfriend help/advice??

So I spent the night over myboyfriend house, and we were planning to go over to diddy riese and the pier. He did know where diddy riese was so he had me mapquest it on his computer. So I got on mapquest then an aim comes up I think he has it on automatic sign-in so when he gets on the computer it loads up. Anyways so an aim comes in saying "Hey boo I missed you yesterday" I wanted to see if it was maybe just one of the girls that mess with him or who she was so I said I missed you too, then the chick went on to say that she can't wait to see me tonight. I didn't even reply. I asked my boyfriend if he was doing anything tonight and he said hanging with his frat-brothers. Me and my boyfriend were suppose to go to the diddy riese and pier but I just went home after he said that. I want to tell him I know but at the same time I don't know how to go about it because I shouldn't have read his ims, let alone replyed back. She was obviously waiting for him to sign on though so she could be the first one to say something. Uh I don't know what to do. One of my girls think we should go catch him in the act, but I don't think I want to follow my boyfriend to see him meeting some other girl. I wouldn't be able to control myself in that situation, I know its not the girls fault because my broyfriend had to do or say something for her to think he was single, but I don't know if I'd be able to control myself from not hitting both of them.