Why is it that when i bend down my knee gives in and its hard to stand back up? i can't bend it or anything.?

Im only 14 and I've taken in consideration with growing pains or whatever but its been off and on every week or so. Its also been pretty bad for the last couple of days. I can't really bend it or move it at all without pain. Im very athletic and im going into high school soon so i'll be signing up for track and i'm worried that maybe when i start running i might pull something. I know a lot about treatments because i've seen a doctor about this but whenever I try what they say to do it doesn't help. I elevate it often and ice it too. I can also move my kneecap inside my skin. My friends get freaked out by it but it doesn't hurt. I mean, like, I can REALLY move it. A lot. Its not dislocated and it doesn't pop out but I can move it from side to side. Does this have to do with my knee pain?