Why is he being cold towards me?

This guy and I had a past of flirting and dating for a whole month ..but stopped because he said he was over me and we should move on.Its been 2 months that we stopped all that . But now hes in all my classes and he wont look at me when I speak to him, doesnt smile and gets all serious around me. He's a outgoing guy and is very cheerful with everybody but me. I felt he didnt want anything to do with me so I ignored him and mirrored him basically being cold etc etc. After class he sent me a text saying its nice to say hi after class. Now that made me feel he does want to be friends? So i went back to talking to him normally but he's still being serious around me and hardly looks at me when talking. Any ideas why he's being like this?