Have you heard that saying: You should talk only good things about dead people and exes?
But if he constantly repeats that his ex was a bitch and that she is guilty of their break-up, there is a chance that he is maybe not over her yet and that you are just a rebound girl to him.
Also, if he talks like that about her, he will talk about you in the same way. He simply can’t stand the fact that he was the one who messed things up!