Why does she ignore me?

How well do you know her? Is she in a relationship maybe? Or seeing someone, there might be a reason why she keeps acting so erratic, maybe she feels uncomfortable flirting (IF) she's taken. The best you can do is just go up and talk to her although it may not be easy, figure out what's going on with her. It might not be easy to be honest and tell her what you're feeling, but it would probably help immensely.