Student Exchange Programs

My question is about this girl I met recently in a class of mine. On the first day, she just turned over to me and was smiling at me when I looked over. From that point on, she would do things like look me in the eye for uncomfortably long times or blush or mostly just smile at me a lot when I was near her. But, even though I liked her a lot (she is HOT), I decided to talk to tell her I had other commitments (getting into medschool). After I did, she told me she did not even like me and would either run away from me or just act really pissed of when she saw me. She would also give me this ’sad face’ look or just stare at me with a blank face at other times. Her friends also gave me this ‘what are you thinking ?’ look.
What I am wondering about is this: I KNOW I am not that handsome, and there was a swedish exchange student in my class who was really handsome as well as a few other guys. Why did she decide to flirt with me ? Was she using me ? If so, why would she even care if I was not interested in her, assuming she did actually get annoyed by my response ?