I need guy advice...

Okay, so I MAJORLY need some guy advice here. There's this one guy that I really like, and we talk a lot and we're pretty friendly. And for the past month or two, I thought that he liked me, because he always seemed to be flirting with me. And then we were working together in class and one of his friends made some sort of sarcastic comment like 'Oh, isn't that cute' or something and then he's like 'But aren't you going out with someone?' and so the guy I like is like 'yeah'. And so my day was absolutely ruined. Actually, my week. But here's the confusing part. I think they've been going out for a little while, but he's still been flirting with me. And the whole me finding out that he was going out with someone was yesterday, but today it seemed like he was totally into me. I am so confused and I just don't know what to think. Is it all in my head? Does he like me? Does he just want to be friends? I have no idea. I would be SO grateful if someone would care to drop me a piece of advice. It would be enormously appreciated!