Carol is right. You should indeed see your regular doctor and let him rule out other possibilities such potassium or calcium imbalance first. It would be dangerous not to, especially since a simple blood test will clear up so many questions. Cramps caused by such imbalances are also extremely painful and you don't want to put your health at risk by assuming it definitely is related to your back problems. When cramps hit me during therapy on my prior fusions, I also assumed it was related to my back and the surgery. After this happened on two visits the PT insisted I see my regular doc before my next appointment. Good thing, as it wasn't the back causing those cramps.
Hope you will check it out ASAP and if it is the back, then it can be dealt with properly.