Nocturnal Muscle Cramps?

Im a 16 year old guy, in the last month or two i have been awaking in the night with terible cramps, it start at the top of the calf and goes to the ankle, the pain is tremendous and i am sometimes afraid to go to sleep because of this, i need to know why im getting these craps in my leg and how i can get rid of or prevent this. Any help would be very very appreciated, thanks