Have you tried sleeping with a pillow under your belly?? Between your knees is good but a pillow under belly would be a good idea too to keep your back straighter.

A few weeks back I woke up with really bad back pain and then on the weekend after I went out and bought a body pillow and the pillow is the best!! Its tucked under my belly and I have my leg on top of it and I am pretty much straight. Only prob is I sometimes wake up to my shoulder aching!