What is this girl's problem?

I'm in 7th grade, and I am generally well-liked with no enemies, which is nice. Except for this one girl, who is a year older than me, and is a "*****" to everyone. Some things she does is giving people the "shoulder" (like purposely bump in to them), glare or point and laugh for no reason, and put you down in whatever way she can, like yelling across the room, "HEY! WHY ARE YOU SITTING THERE? GO MOVE!". She does all of these things to me in particular, and she also "talks" about me behind my back, even though I have only talked to her once, where I was insulted and I simply just said, "cya later" and acted nice. What is her problem?!?!