People in this world are like that. They try to get by in life by bribing people or threatening them in exchange for answers, food, etc. In your case, I'd say she's probably a little insecure, or scared/worried that she isn't smart enough to handle her grades class/homw work on her own.
And she was taking it out on you. I personally (not to start any disagreement problems), believe that you can't judge someone until you meet them. So maybe if you asked her, and make her feel at home and welcome in your prescence, then maybe she'll open up.
There are a lot of people like that who feel as if they can't open up to people special to them because they're afraid of rejection, or just flat out insecurity. But anyway, let me wrap this up. Just talk to her, let her in, and then maybe vice versa will occur.
Remember to be nice about it! That's always the wise descision! I hope this helped!