Is he jealous of my ex-boyfriend?

I had a thing going on with a guy i guess. I don't know if he liked me but we always talked, in classes and such. He went around teasing me and stuff. But then on friday my "friend" asked me in front of him "is it really true that, that Lukas guy was your boyfriend?", after that we haven't talked that much. He doesn't start our conversations anymore like before and if i try to look in his eyes when we talk he suddenly turns his head, pretending that he saw something. Is he jealous?