What is causing my knee to lock up?

If I cross my legs a certain way or go from kneeling to sitting with my legs folded up next to me on one side, it causes my right knee to twist in a way that locks it up and will barely move. It is extremely painful, and trying to untwist my knee has at times literally brought me to tears. When it moves back into place, I see what appears to be a large ligament move and my knee pops, then everything is okay again. What could be causing this? I'm barely 18, so it's not age related, and both my knees are otherwise pain free.