QUOTE(Sakiri @ Nov 9 2020, 03:55 AM)

I'm pretty sure my issue is the sacroiliac joint. It doesn't get enough movement, so it's inflamed and sore. Sitting all day and not getting exercise will cause this. It's a common desk job injury.
The pain radiates across my lower back and around the hips. Chiropractic manipulation helps, as does anti inflammatories, but I cant take those all the time and I have yet to find a chiro here much less see one. I used to go in once every three to four weeks. It was fabulous.
Hmmm....I wonder if this sacroiliac joint is the source of my woes? My lying down pains began circa 2012/13 when, out of the blue while sitting at the computer, I was suddenly hit by excruciating pain in my rump. Could sit down only with a cushion of several soft fluffy pillows beneath me (with considerable pain). At the same time I found that lying on my back results in severe pain again centered in my rump, both extending to my lower back and partway down my thighs. I bought a relatively inexpensive pursue-built chair cushion that, used with one soft pillow, reduced sitting-pain to tolerable levels. I was eventually able to dispense with the pillow and rely on the cushion alone. I still use it on my computer-desk chair. I not sit relatively but not totally pain free. What small sitting pain I have not is usually totally ignorable. This was no quick process. It took years.
As of lying down, that's been even slower. Only early this year (2020) did I realize that I can now sometime lie on my back relatively pain free for an extended period of time. How long a period I do not know, because other body pains kick in that force me on my side(s). I suspect some of this is due to my mattress and sofa, the only devices I have to lie down on. I can't test this theory at, say, a mattress showroom, since I've need to lie on a mattress hours a day several days in a row to form a valid verdict.
The point of all this meandering is that my initial sitting/lying issues began remarkably like what you describe, and only later morphed into what I endure now.