Kit, there is something called Restless Leg Syndrome, not sure what causes it but as you said you 'can't help but move your legs'. It's not painful, just an urge that builds if you don't 'move' (closest way to describe it is that it's kind of like an itch that builds if you don't scratch). Most often, I feel it right before going to sleep. Also, if i'm sitting in the same position for a long time (at a movie theatre)...
Awbrey, not sure if this helps because I've never found it to be painful, just annoying and uncomfortable.