What Does Severe Pain In The Left Knee Indicate?

I have serious pain in my left knee but have no clue what I did to hurt it. I don t remember twisting it, popping it, nothing. But severe pain every time I moved in bed woke me up. Even as simple as rolling over. Then to walk is very painful. Sometimes it feels as the leg wants to bend the other way when I walk. I bought a brace. It was very painful in one spot around the kneecap for about twenty minutes but that pain went away. Other sharp pain when I walked with the brace in place only happens when moving the leg in certain directions. Pain is on my left hand side of the left knee cap. Hurts to touch in one spot. After initial use, the pain seems to fade until you move wrong. Then it comes back with a vengeance. But feels MUCH better with brace on than with it off. Taking breaks between walking only causes it to stiffen up and I have to battle through again till it loosens up. Any ideas what it could be?