you may have an annular tear in one of your discs. Specifically l5-s1. Conclusively they would need to do a discograph.

It is not well known that annular tears may leak fluid from the nucleus and this then toxically intereacts with the nerve and creats pain. I believe I suffer from this as well after I had a microdiscectomy performed at l5-S1.

Avoid sitting for over 15mins. Avoid standing also if it causes pain. Take 1gram of glucosamine in the morning with food and another 2grams at night with food. Do this for at least 3 months. I reckon you will see an improvement. As I've been doing this for that long and it is getting better.

If you can't stand the pain. and thinking about surgery. Find a surgeon or GP who recognises the fact that you can get chemical leakage from a tear and get them to perform a discograph. They will inject die into some of your discs to measure pressure and leakage. Then they can identify your disc and they may discuss fusion or ADR. If you have a fusion be sure they take out the offending disc don't let them fuse without removing the disc, as it will just keep leaking...