This position helps me a lot when I sit, it takes the stress off my low back.

http://hab-it.com/blog/?p=105

My SI joint issues started when I bent, from my waist not my hips, over to pick up an empty milk jug off the floor. It locked up, I couldn't straighten up. I don't know why I have this problem but some theories I've heard from PT's is that it was from an injury many years ago or it's an inherited condition. My PT also thought with good lower back mobility and neutral spine I can keep it under control.

I hear you about the cat. When I had injuries that I needed to support on a pillow my cat used to steal the pillow. I would wake up in pain and find his little head snuggled in the middle of the pillow. It was an airline type pillow and it was the perfect size for him. I swear he had a smile on his face.