Cramp. Try to get more Potassium as others have noted.

Now, to help with the cramp while you're in bed: Let's say that the inner left leg is cramping. Lay on your left side and try to press your left knee into the mattress enough to engage the outer leg muscles but not enough to lift your body off the mattress. Hold this for 6 seconds and release. Do this a couple more times. This is reciprocal inhibition and when you contract the opposing (antogonist) muscles against a non-moveable resistance, the cramping muscle starts getting urgent messages from the brain to release so that your leg will move. You're tricking your brain into forcing the cramping muscle to let go. If your electrolytes are really out of balance, this will only be a temporary fix, but when you're screaming in pain, temporary is often good enough.