Hey Patrick! The cause of your cramps may be completely unrelated to your previous back problem. Are you keeping hydrated while you swim? I make sure to drink water before I swim and keep a bottle handy to drink from throughout my workout. And, depending on the conditions and workout, I will drink Gatorade (or something similar), as well.
You may also have a potassium defiency which can lead to cramping. It wouldn't hurt to eat a banana before you swim and see if that helps.
How cold is the water you are swimming in? As much as I love swimming in cooler water, my legs will cramp in anything cooler than 79 degrees, if I don't keep moving.
As you know, I am not a doctor, but I believe my suggestions are valid. If none of this applies to your situation, you may want to check with your doctor for advice.
Good luck and see you at the next meet!