Is it normal that i get charlie horses (cramps in my calf) so often?

I am a dancer and have been for almost 6 years now. I dance for 1-2 hours every week from Monday-Thursday. Almost every night, or every other week, I'll wake up in the middile of the night with a horrible cramp in my calf. Sometimes even in both of my calves. It seems to go on forever, and I don't know what to do about it when it happens. Usually I just shout in pain while it's going on.
Anyways, my point of this was that I don't understand why this happens to me so frequently. Am I the only one going through this? Does this happen to anyone else on this site? Is this normal? (If you have any tips, please let me know. Thank you!)
Xx Jennifer