Part-Time Bitch

A girl who is nice sometimes, and is a bitch other times. Can also refer to someone who is a bitch to some people, and nice to others.

Person 1: "Did you see what Celia did? She's such a bitch!"

Person 2: "That's weird, she's so nice to me!"

Person 1: "Huh, I guess she's just a part-time bitch."