Why doesn't she like me anymore?

Hey, I'm 16 years old and in high school. so there's this girl who I used to talk to all the time, she would invite me over to her house and we'd talk in the hallways. we used to talk on facebook until about 1am. I was considering asking her out, but I never got around to it, then over the holidays she met this other guy and is going out with him now. we still talked to each other after that, but since a few weeks ago she's been really cold to me, she gives me evils when I see her in the corridor, and snaps at me if I whistle or anything. can anyone tell me why and how I can get back to being in with her.