Leg cramp

My leg cramped up early in this morning. During the phenomenon, I couldn't move. All I could do was stretching leg fingers and waiting it go away. I screamed in my heart "Please stop !" during the phenomenon. But, it was continued for some minutes.
It happens about every a few years. I think it happens when I'm tired. I want my job be reduced. But, I'm working now in my office despite it is Saturday today. I may work tomorrow, too.