Take a look at the thread on salt in my signature. The usual culprits are minerals like magnesium, potassium, sometimes calcium, but the key is salt. If you don't have enough salt those minerals will get flushed away. Salt gives your body kidney velcro, so to speak, so that the minerals will be reused instead of flushed down the toilet. Low carb diets tend to flush the salt out of your system, so make sure you replace it.

I usually find that a quick fix of potassium, magnesium, and salt gets the cramps squared away.