Am I a rebound girlfriend or not?

My boyfriend (we just started going out) broke up with ex-girlfriend right before he started dating me. He's still living with her (moving out soon of course) but nevertheless you can tell why I'm nervous. Not just that but he brings her up in conversation. I can't deny I feel awful when he does that. I think I'm really selfish for feeling this way and maybe it's in my head. I really want to know though am I a rebound girlfriend or not?