What should I do about my friend?

Normally, I am not one to blame. However, I was at my friends last night who knows that I have bad back pain. She was with another girl who I didnt even know and I was kinda like the third wheel (in friendship terms) because they were playing xbox and I was just trying to go to sleep because they had been playing for like 3 hours. Anyways. I wasn't asleep when they were done playing, but they thought I was because I had my eyes closed. They started by slapping my face, pinching me, ect. It wasn't that bad, and I pretended to be asleep because if they knew I was awake it would only get worse. So anyways then my friend sits on my back and hits it really hard. To her it made a "funny noise" So she kept hitting me. I told her "Dude I have terrible back pain!" But she just kept doing it knowing I was awake. I feel like my friend is actually a little messed up because she's pushed me over a stairwell, and slapped my fingers in a door on purpose, but not my back actually hurts. I have problems with my back and my knees and I feel like I actually injured it because every time she pressed down my pack cracked even more. I just don't know what I should do? Should I tell my mom? She sometimes over reacts about this stuff but I feel like I need to go to the doctor. Is there any exercises that will help with my pain? And what should I do about my friend? Btw, Im 13 and a girl.