I know exactly how you feel, Ive been through it a few times before. It could be jealousy or low self esteem or it could be that they are just down right mean. Either way you need to be strong and like the other person said confront them and or ignore them to a certain point. Showing them that they will not affect you anymore usually makes them back off. Good luck
There could be several reasons why this girl is behaving like for instance, it could be out of jealousy. It could also be as a way to make herself feel better as she may have a hidden low self esteem. You should consider confronting her about it or choose to ignore her.