My friend seems to be mad at me, even though I didn't do anything to him?

[Courtney]

To give you some insight, we kind of have a history. We used to talk all the time and he admitted he liked me, but I turned him down because I was seeing this other boy (who I'm still dating).
I thought we were passed that and could be friends still, but he a couple months ago he told me he hates losing and feels like he lost. But we were still on good terms.
But this week hes been going through something I guess because hes been absolutely irritable and won't even look at me. He bitched me out on twitter for making a joke, and completely ignores me at school.
What should I do? I'm sad that hes acting this way, because hes a good friend and I do care about him.