Knee in pain? Don't know why!!?

You may have slept wrong on it. That happens sometimes. But years ago, I developed a pain in my knee for no reason all of a sudden and it swelled up and hurt really bad. Come to find out, I had aggravated it at some point and the synovial fluid swelled it up really bad. I just kept ice on it for a while and kept moving it around until it worked itself out. Good luck, I hope you feel better soon, If the pain doesn't go away, I would slide down the stairs and tell my parents.