When sitting almost anywhere, I have gotten severe pain in my sacroiliac area and then the muscles start spasming and the spasms then travel to my lower back. I also get bad hip pain if I lean forward too far.
It does depend alot on what I'm sitting on, as well. I have a coccyx cushion that I use in school and at my dinner table, which helps a little bit. But my pain got so severe, I had to talk to my PM about it. He worked with me and I put me on several meds (Lyrica, Soma, and Ultram) so that I could get through the day in class. But the chairs in class are truly terrible..even people without any back or sacral problems get uncomfortable in those chairs. But I have several cushions, a lumbar support cushion, and a small foot-stool that I bought. The combo of the meds and my cushions helps my pain alot, though some days it hurts more anyway (epecially with storms or humidity).
I am not sure, though, why I have this pain. I have a history of severe spinal stenosis, for which I've already had surgery. I do have alot of scar tissue, which causes problems. BUt I don't know why my sacral, hips, and pelvic girdle hurt so bad. Laying down, like in a recliner, also really relieves the pain.