He doesn't like me now?

I have this friend, he's my classmate. I've known him for at least a year, but we get along really well together. We play games, beat up each other (playfully, of course) and call each other names. He'd call me fat, shorty, midget and the like. It never bothers me when he does it, though. Besides, compared to him, of course I'd look like a dwarf, he's like six foot tall. And I know that I was not and am not fat. (100 lbs at 5 foot. Okay for an asian, so no, I'm not stick-thin)

Then came a new semester. He began to shy away. I don't think I've ever said anything to hurt his feelings. I mean, we still talk, but when I gave him a punch(painless, I assure you), he doesn't punch back. And I know he didn't "grow out of it"--he's still jumping around and punching his other guy friends. Plus, when we talk, most of the time he's staring at the floor, or desk, or whatever is convenient. And he had this new habit of putting his hand on my shoulder, but it didn't last very long.

Last week we had a class photoshoot, so we all dressed up and stuff. When the photos were done, some of us helped to carry the chairs back to class. He helped, too. Now, I don't mean to boast but this part really bothered me. This group of guys from another class passed by and told me I was "hot". I told them thanks (real smooth, I know). But HE just sped up after that. Not a single word, he just suddenly left me behind.

So, I'm sorry for the long story, but I figured I should explain a few things first. Also, I believe he doesn't have a crush or me or is romantically interested in me. I mean, I don't think he's like that. I thought of asking him directly, but I feel that it's better I don't. I'm not close with his friends and my group of friends figure he has a crush on me--which, as I said, I don't think so.

But why is he suddenly so... I don't know, not the usual with me anymore? Is this a normal guy behavior?