What Causes Sharp Pain, Swelling And Difficulty In Bending Knees?

after getting up off the floor at work my knee got a sharp pain in it and i had to straighten my leg to make it comfortable. That was a month ago and it still is very uncomfortable. It's hard to bend my leg and it is swollen. I've had an x-ray, but have heard nothing from my doc, so I assume it's not a break.