Hi, You have not mentioned the other details. We give you some information. Muscle cramps may be caused due to a variety of reasons but the most common cause of muscle cramps is not fatigue, dehydration and when there is reduced blood circulation in the legs. Dehydration, reduced magnesium, potassium or calcium in your diet,pregnancy,excercise, alcohol, thyroid disorders, kidney failure, some medicines like diuretics may also cause it. It lasts briefly from a few seconds to few minutes and resolves by slowing stretching the affected muscle. If it does not resolve even after that or is very severe or debilitating then you need to visit a doctor immediately.