Don't like to be treated this way.

So I asked a girl at work a question like "have you seen this so and so paper?", she didn't answer, just stared at me with a pokerface. I asked her "oh, are you post-duty?", and she continued to stare at me with a poker face, then proceeded to chat with her friend. I just walked away.
I don't understand. She was finally treating me just fine, saying hello and stuff. And then today she suddenly decided to go back to her bully ways. I just don't understand what I keep doing wrong. It's like....I'm supposed to be nice to them regardless. Otherwise, I'm the bad guy. And I don't know how to undo this. It doesn't make any sense. They can treat me like shit and still maintain rapport, while I can't treat them the way they treat me because then it will give them a reason to bully me even more. It just.....why me?