make sure you're getting enough potassium. bananas are good for this, but if you are already taking a daily multivitamin you should be covered. I would make sure your sodium intake isn't too high.
if you work a desk job, its probably just normal muscle crampign due to lack of daily use, but then extra tensing of the muscles while you workout. stretch them some once you have warmed up/done a couple sets.
I would also do some kind of calf exercise. I have genetically large, strong calves. Since I was 15 I could walk up to most standing calf raise machines, drop the pin at the bottom of the stack, and rep to 20.
Therefore, I only work them every other leg workout, and only 3-4 total sets, pretty much rotating from donkey, standing and seated calf raises each workout.