Eat a tums or 2 with each meal. I had the same thing and it turned out i wasnt getting enough calcium in my diet.
Simple fatigue can cause muscles to twitch, and clumsiness in people your age is very common as the body grows too fast for the brain to keep track of it.
If you can actually see the muscles moving in little waves under the skin - when the leg's still - then see your doctor. Otherwise I wouldn't worry about it.