If you weren't letting on that you liked her, maybe she just figured that all her flirting with you (sitting really close to you etc.) was not working and that you weren't interested, so she backed off.
Quit playing hard to get, and ask her out already. Girls love being asked out. And you're not asking her to be your steady gf or to marry you or anything. Just a date. No big deal. To use an appropriate cliche, just do it!
Wanna go out?
Sure, but since you're in CA and I'm in MA, I guess we'll have to meet in Kansas or something.