There's plenty of possible reasons what, why etc...
She could:
1) Not be interested and be a psycho leading you on, just so she can be rude when you talk to her (not likely)
2) She could like you too much, and so when you do talk to her, she gets shy and doesn't know how to act, so falls back to being withdrawn (you can perceive that as rude and bitter)
3) She is a rude and bitter person all the time
4) It's you, you're just perceiving her wrong...
There's a billion possibilites.
But there's just one fact:
The only way to know for sure is if she doesn't reject you
Yeah, it's kinda ironic. But the only way to know is if you make the move. Ask her out (point blank, not in vague terms), and see what happens. Ask her out more than once. 2-3 times is enough. And this is only in case she likes you so much that she's playing hard to get.