Abusive friend? Help!

Ok so my friend, I'll call her Sophie, has been acting really weird and agressive. So I will start from the begenning of this year when we really became friends. She seemed pretty nice and I had known her from last year so we started hanging out with each other. I started to trust her and everything and she became kind of like my at school best friend. But then one day I had just had peanut butter crackers and I was really Hungary so I went and grabbed one of her chips ( I was going to give her something else in return) but I had forgotten that she was allergic to peanut butter and so as I did this she grabs my arm digs her nails in an starts yelling at me half way out of her chair to the point where I was almost scared. So then after that it was like everything had gone back to normal and we didn't really talk the rest of lunch. So I had  not really thought too much about it because it was my fault. So another time this happens but worse and I was upset about this one. So she had said something about me and I said something  like "well at least I'm not mean" or some crap like that as a joke because we were joking around with each other and she grabs both my arms to the point where she was ripping my skin and told me do you want to go bitch or something like that. And if you saw the look in her eyes😬😬😬. So I said let go of me and I tried to escape her grip but she just tightened and told me that I was weak and that she knows she can beat my ass. So I was like I'm not weak and I could beat her if I wanted to ( btw I am a very calm person so I kind of have a "shy" voice) and she said try me and my friend saw this and she didn't do anything but raise her eyebrows and smile. So then she let go and once again everything went back to normal. Then today I knocked the music off the stand in orchestra just to joke around and her face just goes blank and she shrugged and then she grabs me again and tells me that no one likes me and I have no friends etc in a really harsh voice and all I said was dang and put it off again but I really want to tell her off but I don't know how to in an assertive way. Should I give her another chance and if she does it again not be her friend or just end it now? She has a sister in special ed could she have a disorder?