Those are called "Charlie Horses". It's when your calf cramps up and it's painful if you don't do anything right away. There is a method to stop it. As soon as you get it, straighten your leg and put your foot in the upright (perpendicular) position. What I mean is, pretend to be standing up straight. If you can't do it while laying down, get up and try it then. When you don't move your leg for a long time, it sometimes cramps up. It's because your leg's bent and your foot is too relaxed. When you move, the whole thing tenses up and that's what gives you the cramp. Straightening everything out will stretch the muscles before it cramps up more. That's what stops the pain. The sooner you do this, the less likely your cramp will become a "sprain". If you don't and it gets worse, then you'll have a hard time walking later.
