See a spine surgeon they’re quite helpful.
I'm trying to figure this out myself. My rheumatologist says he doesn't think that the flares come on strongly to the point where I can't walk needing a wheelchair or need a walker if I can put pressure on the affected leg. When I lay down, I do feel the need to move my pelvis, and I am constantly tossing and turning at night. My SI joints are affected (errosions, inflammation, and one side narrowing).