Can anyone give me an explanation for what happened at homecoming?

For some background information, there's a boy in my grade who I've known for about five years. He's a bit shy, but we've always been really friendly with each other and I've become close with some of his other friends. We had Pre-AP US History together last year, so this guy and I started talking a lot more than we did before. However, in the beginning of the school year I started noticing him starring at me...a lot. When I dressed up as a 1920s flapper for Halloween, he refused to meet my eye for the entire day. A little over a week ago I'd realized that I had developed a crush on him, so the constant starring has become a bit more awkward for me.
Anyway, last night was the homecoming dance, and I became really excited when I found out that he was going to be there. We would have a four day long weekend afterwards(which started today), so I wanted to spend time with him beforehand. I dressed up really nice and spent the night dancing, walking around, and talking to different people. I never saw him until near the end of the night. He was talking to one of his friends when I walked by. He looked over at me in surprise and whispered something to a guy he was talking to. They both looked at me. The guy starred for a few seconds before looking away, while his gaze followed me as I walked across the room with my friends. A few minutes later, I broke away from my usual group to hang out with the guys and girls from our Ap US History class, and all of the girls started dancing the same dance. As I was dancing, out of the corner of my eye I saw him standing behind me, blankly starring again. I turned around and grinned,
"Hey Spins!" (AKA his nickname)
"Hey." He said. This was weird because usually he greets me with a smile and says my name when he says hi, but last night he just raised his eyebrows with a weird smile and then looked away.
"Are you enjoying the dance?" I asked him.
"Meh." Then he walked away from me. He's never acted like that towards me before; he's usually so nice. A guy we're both friends started talking to me with an even more friendly tone than he was using before when I had first entered the group. I think he noticed that I was hurt by that. I walked away from the group a few minutes later, and he was just sitting on the bleachers by himself when I left. I never saw if he started dancing with the group or got back up after I left.

Can anyone offer a possibility as to why he acted that way?

Who knows? Teenage boys are awkward when it comes to girls. It sounds like hes afraid to be rejected. Why not just start a rumour that you like him and see what happens? It worked for us when we were that age

I appreciate the input :) I mostly just hope I didn't offend him somehow...

You didn't even talk to him except to say "Hi" and "enjoying the dance?" How could that semi-conversation offend him?

You were rather involved in interacting with everyone but him. If you actually like him, why not talk to him more? If he rejects you or is rude to you then at least you'll know that it's futile to be crushing on someone who doesn't reciprocate or is an asshole.

UPDATE FOR ANYONE CURIOUS

So the long weekend I mentioned before ended. I walked into our APUSH class, and he was already sitting down (he sits next to me). He saw me coming and put his head down, starring at me with a little bit of a sad look. I'm still not over him just walking away from me like that, but I took the advice that both of you gave. I grinned at him as I sat down and said hi again. This time, his head popped up, his eyes lit up, and he grinned back at me,
"Hey! How are you? How was your weekend? What'd you do?"
Then after our conversation, one of my friends brought up another guy who tried to flirt with me that weekend before I walked away, and I saw him get a bit angry X_X
I'm so glad he's back to normal. Still not entirely sure why he acted that way that night...but it's good that I have some ideas now.