I was recently playing basketball at my practice, and I was going up for a rebound. I went up to jump and when I landed I was having really bad pains in my knee. Ever since then I have been having trouble walking, and keeping balance on my leg. I constantly keep getting sharp aches up my leg from my ankle, to the back of my knee. I can barely touch around the knee cap, or the back of the knee. I can't bend my leg, and it hurts to walk up stairs.. What's wrong?