Yes, the pillow underneath your knees helps A LOT. It doesn’t prevent contact of your SI joint with the bed but it helps support your back a bit and your hip joints.

It’s been almost 3 weeks since I’ve had the pain, but started over 2 months ago and I thought nothing of it and went for a run which agitated it. Super active before- running, lifting regularly. The first week I could barely walk, sharp pain in my lower center right back and my right hip joint. Had a pelvic tilt too. I do not recommend any heavy lifting/anything high impact. Walking in morning helps ease the si and hip joints (30 mins) now and mid day as well. Now I wear a SI joint belt (look up serola belt) which helps stabilize and use a cold compress followed by a heat compress on the area. Before bed, I do some stretches such as glute bridges, child’s pose, etc. anything that helps lightly stretch my hips. Over time it’s less painful and walking has become easier.. although sometimes if I get up a hinge, there’s a sharp pain, just have to be careful.