Charley Horse What causes it?

What is it that causes a lower leg muscle pain, a charley horse? I have gotten them at times and my leg muscle will get as hard as a rock, after it goes away my leg will be sore. When I would get them was when I would use regular swim fins and start out doing too much without a warm up. One swimmer told me they were from a low sugar level. I have the Zoomer fins also, and have not had a charley horse with them. Merry Christmas!