Interesting that you mention this...I haven't had bad cramps in the last few months, but before that, on and off, I would get cramps in my calf, and also cramps in my feet...basically in the muscles at the bottom of my feet. Both very painful...and in the case of my calves cramping, they would be very sore for days afterwards.
For a little background, I have fairly significant herniations at l4-l5 and l5-s1. These were only diagnosed about 6 months ago, however, I've had backpain for many years, and I've had these cramps for the last 5-6 years. I don't know if the herniations and the cramps are related. I would imagine that at least indirectly they are, due to the tight muscles in the back/legs, etc. Maybe there is even a direct relationship with the nerves being pinched, but I don't really know. Regardless of what the cause is, I would agree with some of the advice above. Stay hydrated, get more potassium (I think bananas are a good source), and stretch out your calf/leg muscles.