Whats wrong with my knee? help!!!?

Well you could have just tweaked the crap out of it or you could have torn something. I wouldn't be able to tell without doing a physical examination of your knee, seeing as I am unable to do this. You should go get it looked at by a doctor. While you're waiting Rest Ice Compression Elevation(RICE, look it up for more info)is probably going to be your best friend. Hope this helps.