I've done something to my knee. I was sitting down and my foot slid forward. Sudden excruciating pain in the back of my knee, to the left hand side of my left knee. I went clammy and nearly vomited from the pain. Pain has subsided if I'm resting, but knee feels kind of "throbby". There's a bulge at the back of the joint. If I put weight on it I get sharp pain if I don't walk straight on a flat surface. Any ideas what I've done?