Do you think this girl liked me? + Why would she stop talking to me all the sudden?

Yes, she definitely liked you! My guess is she isn't talking to you anymore because she feels if she does she is almost "cheating" on her boyfriend. Silly, I know. But since you two obviously had a fling she is probably trying to make this new relationship works, and that means trying to forget about any other guy she liked and focus on the boyfriend. Horrible situation. I would either call her and ask if you can get together because you want to talk something over with her. Explain how you feel, and see what she says.OR just wait it out. Hopefully this new relationship won't last.