I have the same problem, finally after having an MRI it turned out to be a herniated disc. When I sit, my upper body weight compresses on the disc causing it to bulge more, hitting the sciatic nerve, thus causing severe pain. The first approach to treating this was Motrin (ibuprofen), occasional vicodin, and physical therapy. It is gradually getting better for me but is still there. This has been the longest lasting pain I have ever endured, and I've had many surgeries that have healed faster than this.