Did you lift anything heavy? What kind of pain is it, and is it primarily in your knee or is it in your back and shoots down your leg to your knee? It could be a ruptured disc, but if you didn't lift anything heavy, or do some ridiculous bending, it probably isn't that. But if it is, you should get it checked out, cuz I had that and it was uber-painful for a while. Surgery made it better but I will forever have a bad back. BLAH!