What Causes Pain And Stiffness In Knee?

Hi Doc, I have a habit of letting my leg hang out over the bed when I sleep on my stomach. The last time it happened, 2 days ago I felt pain in the back of my knee and i had problems straightening my knee. I did collapse on me a couple of times the following day a couple of times. I iced it and rested it, and my knee feels normal. How bad do you think i injured it? and should I need to worry?