This just happened to me too Wednesday night, woke me up from a light sleep in severe pain in my calf, had to stretch it out as indicated here to relieve the pain. It has happened in the past as well on occasion. I find it happens when I go strict. I ate a banana the next morning in case it was potassium related but suspected salt intake as Tarlach indicated.
You'll never see the light if you're in someone else's shadow, or said another way, life is like a dog sled team, if you're not the lead dog, the scenery never changes