What should I do about my friend?

Ok well I have this one "friend". I've known her since i was about 5, I guess. Well I'm in middle school and I haven't been in one of her classes since Kindergarten until this year. So lately I've noticed that she's been really mean and off the walls. She takes my stuff and when I ask for it she yells at me. She throws food at me (well sorta) at lunch. She smeared applesauce all over my face. She hits me and stabs me with pens and pencils. One time it even left a mark. It was a couple days ago...last Wednesday, and I still have a scratch on my arm from it. Me, her, and a two of my other friends were hanging out yesterday and we were on top of this one hill because that's like our meeting place. Well when we got there we decided to play king of the hill (two of us start at the bottom. Two at the top and the two at the top have to stop the other two from having one body part on this block of cement for 10 seconds) and Allison and I were at the top guarding. So Megan (the friend that I'm talking about) come and just plows into me! I said WTH! and a couple other nasty words...anyway when me and Allison (my best friend since I was three, lives across the street, we call each other sisters...yeah) were the ones at the bottom of the hill. So we try and get up and I'm about to go lay on the cement so we win, and I get on the block then she plops herself down on top of me not a care in the world. Right on my shoulder. And I don't know if it was just me but I heard it crack. It's fine (my shoulder) but she always asks me if I'm mad at her and I say no. I really don't know what to say to her anymore. Like today in music class it was the end of third quarter so we watched a movie. Well I have a book project due on Monday and I didn't finish the book so I started reading it. She starts taking my stuff and my book. Closing my book so I lose my page. And at one point she tried to take my shoe off! What's up with that! She has her period but as far as I'm concerned she's not on it at the moment. So I don't know what her problem is or what I should do. Please help.