She used YOU by studying with you. Girls nowadays KNOW that if their are any smart guys in the class especially UNAWARE idiots all they have to do is flirt with them a little bit and BAM!!! game on. We guys tend to ASSUME that if the girl is giving us any sort of attention it MUST be that she likes us so it's no surprise and girls are masters at catering to our egos. They send ALL the right signals KINO, eye-contact, jokes, etc etc and we fall for it. The best way to weed these chicks out is to ASK them out to lunch very directly. Fuk all the "work your way to it" because it's a complete waste of time and if the girl is interested and giving you the signals that is the RIGHT signals and has genuine interest their is no reason for her to decline anything simple as that.
Another possibility is that since she MIGHT see you as competition and she is distracting you from ACTUALLY getting any study done by flirting with you, kino and wasting your over-all time. Trust me on this because I've had girls done this to me...JUST so they can get a better grade. It's sad, petty and pathetic at the same time but this is very real. People will do all types of sh** to get ahead and her behavior is very suspicious so I wouldn't be surprised if she has ulterior motives.
Most girls nowadays are all about control but the problem is that they can't balance the amount of control they should apply, so they over-do it. Regardless, pay this chick no mind and don't even look her way anymore. Do NOT ask to study with her because she will fuk with your head and waste your time. Even IF she calls DO NOT pick up her calls, I wouldn't be surprised if she calls you asking if "we" could study. I don't know what it is nowadays but people in universities have become so grimy, even the dudes that all they care about is using people to get ahead so keep an eye open for people like that especially girls like her.
BUT I could be completely WRONG and she could be playing a little hard to get thus, trying to play games. Which in that case act like she doesn't even exist, talk and do your thing with other people. The rest is up for you to figure out.