That can be from a number of causes. Usually they are related to the nervous system. They can be temporary, caused by anxiety, or more permanent, like the ones associated with nerve damage of some...
Some are the reason of muscle fatigue and cramps are Altered neuromuscular control, Dehydration, Electrolyte depletion, Doing a new activity, change in climate.
An over exertion of a muscle causes a charlie horse. Charlie horses mostly occur in the calves of your legs. It is the muscles locking. It should hurt for a little while maybe a day or two but in...