Why does this girl that i dont even know keep bullying me?

Sorry to hear she's doing this to you. I've got the same type of people at my school, they b*tch at you for no reason what so ever, even if you haven't done a thing wrong and then when you are one to one with them alone they give you no bother. My guess is that they are insecure people, maybe she's jealous that you maybe have more friends than her, or if the guy you hugged was someone she liked. But because she's insecure, she won't be confident enough to stand up to you alone, she'll need someone to back her up. To sort this out, I would wait until it's you and her alone, like in the bathroom, and ask her why she's being how she is. She won't have anyone to defend her so she should be honest with you. Hope this helps. :)