I don't have a crush on him, so why did I blush?

The teacher made me sit next to this boy, that I formerly use to have a crush on. (About 5 months ago) I've gotten over him now. When he sat down next to me, everyone was like "ooooh" & "awww" & "look at the happy couple" & "going to sit next to you're girlfriend, are you?". I turned a bright shade of red. Ofcorse, his friends pointed it out and now, them and my friends still think Im crushing on him. How can I prove that Im not, and why did i turn red?