Help Me Find This Actress

I'm trying to think of an actress. She's in a romantic comedy as the bitchy girlfriend of Seth Rogen, Jason Segel Justin Timberlake or another one of those guys. Then a Mila Kunis type girl comes into his life and he leaves the bitchy ex girlfriend. I never realized how many of those types of movies there are. Anyways the bitchy girl is short with black hair and a sort of permanent duck lips? Does anyone know who I'm thinking of?