I Have Very Sharp Persistent Pain In The Back Of

I have very sharp persistent pain in the back of my knee and the outer part of my knee where the tendons meet the bone. It came on gradually and now it is severe pain. Doesn't appear to be any swelling. Just pain. I cannot bend or straighten my leg. Hard to walk and go up and down the stairs. There was some crunching sounds coming from my knee the day before. I do not recall straining, hitting or twisting my knee.