I can't get my boyfriends ex-gf out of my head! What is wrong with me?

My current boyfriend was dating this girl before me, for about 9 months. He hates her now and they don't talk at all but for some reason I can't stop thinking about his time with her. The hardest for me is when he compliments me. For example he will say things like "you are so beautiful" but everytime he says it, his ex girlfriends face pops into my head and I can't help thinking he was saying all these same things to her not that long ago and it loses meaning. Also, she is not very pretty which also makes it lose meaning for me. Is this a common problem that I am having?