Do you have back pain some days and can’t think of ANYTHING you’ve done to aggravate it? In fact, it’s sore when you’ve spent a day sitting on your bum doing nothing?
One of the most common ways to get a passive pelvic sprain is sitting with your legs outstretched, as in a recliner chair or on a couch. The opposite forces of your stretched hamstrings pulling your pelvis backwards, and your flexed spine pulling your sacrum forwards can shift the sacoiliac joint and cause it to lock on one side or the other. After this, you’ll have pain when going from sitting to standing or rolling in bed.
Unfortunately you can’t fix this yourself, it does need assessment and treatment, but you can avoid it happening by sitting with your feet on the ground, and a small cushion supporting the lower back.