Is it normal for my ex to do this?

So not too long ago, when me and my ex were together I cheated on him. My intentions were never to cheat, it just happened. He ended up finding out from somebody else and we broke up. I felt bad and my ex was hurt. We still ended up being friends and after a while we started talking again. We soon went to six flags for a class field trip after getting really good scored on our exam tests and so much happened. We did a bunch of "stuff" and the next morning he changed. He was playing around too much and trying to be funny when he wasn't and was joking around too much and trying to be cool. It made me mad cause just yesterday he was a sweetheart and being all close to me and I've known him for a while and now he's trying to be cool. Maybe he's acting this way cause I cheated on him and he's trying to get back at me but if he was doing that, I think that's childish. I have no idea why he's acting like this. Should I be worried?