fkv,
It sounds like it could be a pinched nerve. I've had spondylolisthesis for 25 years and started having pains in my buttocks, nether region and lower abdomen while sitting about two years ago. It was just random pains, but very sharp and intense. I thought at first my bladder had fallen or uterus prolapsed. I tried various cushions in my chairs at work, home and church. I also developed burning sensations in my legs over the past year, and the pain in my buttocks and hips grew unbearable. When I went to the doctor, I was told that my spondy had progressed and a disc had degenerated.
I also have a very achy pain in my lower back on the left side, mostly when I sit. If this sounds like the pain you're experiencing, I recommend you see an orthopedic doctor and get some x-rays. They will probably want you to get an MRI, too, because it will reveal what's going on with the nerves.