Are you doing the snooker style of stance? If you are tall, it might be hard on your back on a pool table (since it is lower than a snooker table). Some tall snooker players bend both of their legs to lessen the strain on the back, one example is Ricky Walden. You can still keep the basics pretty much the same. I get huge cramps and back spasms with a regular straight legged snooker stance on a pool table, and I'm an average height European guy. The combination of the stiff back leg, and the necessary twisting to get the arm behind the head puts tremedous strain on the lower back for anyone over 180cm on a low pool table.
https://www.youtube.com/watch?v=4Z0nPMzvNqk
You could also do a different stance, a la Jim Rempe, where you spread your legs out wide, but keep them straight. Gets you low and pretty steady. If you allready have back problems, it probably won't be gentle enough on your back, but still better than the orthodox snooker stance.
A third option if your back is allready bad, is to sort of imagine that there is a bar stool behind you and you want to sit down on it, bending both legs and not really bend very far forward. You bend at the hips only as far as you can without any pain in your back. When my back is acting up I find this kind of stance to be the least uncomfortable. It's not orthodox, but it gets you down to where you can see the shot (sort of). Getting your chin on the cue is out of the question, of course. You'll look more like a billiard player than a pool player, but at least your back will not go out. Like all other back advice the trick is to do the work with your legs, not your back, and to do as little twisting as possible.
Whatever your stance is, the key is to position the legs correctly, to have them far enough apart and correctly bent. Having straightish legs close together is just begging for trouble down the road, unless you are very short.