Is she a bully or my friend?

There's this girl in my cl named Megan. She's really overweight and I can tell she's insecure. Anyway I used to think she was shy and didn't like to mingle much but she is SO not! Ever since my other friends inroduced me to her and we started talking more she's becoming meaner and meaner. She's not mean to my other friends, it feels like I'm the only one she picks on. When I'm just walking around with my other friends, she jumps up behind me and pulls my shirt collar(she's REALLY strong) and carries me up. I'm petite and short, so I can see why she chose me as a target but I'm so confused. When I get home from school she starts calling me nonstop. After I say I gotta hang up and do my homework she stalls me. And when I hang up she calls me again and again. Also when we go to the neighborhood park or something she throws foot at my head, and when I wear a new shirt for free dress(i go to a private school) she goes "That's an ugly piece of sh*t." She seems to get mad when she finds out that a guy likes me. And today when it was recess(i'm in 6th grade no comments about that please)she keeps on hitting/punching me. I come home with bruises and I have to tell my parents that I fall alot because I'm scared she'll hurt me again. Please help, how do I get her to stop!? She constantly hares me but not my other friends. I don't know what to do and I had to fake being sick twice because I didn't wanna face her at school. Answer quick please I need some ideas on getting her to stop by tomorrow! Thank you.