I am sorry to hear you can not lay flat,

and I understand what you are feeling, I have been bent over for over a year now, and can not lay flat at all, I was told it is the mussels spasms, and then weakness comes in, I have core weakness also, and my RSD is full body so I do not get the shots, it is way to painful to lay flat, and to get back up is so hard, I have had to roll off tables in the past with PT, and it takes so long to be able to get up as far as I can, it is in my lower back also, then it goes all the way up my spine, my RSD started in my C- spine, and spreed really fast
So I keep pillows all around me they seem to work for me,
But is your back pain from RSD?

I hope things work out for you