Guys (and girls)... Why does my ex treat me like crap when I did nothing wrong?

Okay, first of all we have a 2 year old daughter together so we have to deal with each other. However, when we do deal with each other he's always short or pissy with me. I'm always nice and civil and don't try to start any B.S. with him but he'll get pissy over the smallest little things. Now our marriage ended because he cheated and decided he wanted the other woman so if anyone should be being short with anyone, I think I have more right to be the pissy one here. And yes, he's still with her. He's told me he doesn't want to have "anything and I mean ANYTHING" to do with me. I always let him initiate contact (which isn't much). I don't call and bug him or email him or anything like that. I just don't know what his problem is with me because I don't know that I've done anything to him. Any suggestions why he's such a jerk and what I can do about it or ways to deal with it without getting frustrated??