It could just be you're body and spine adjusting to the new belly and weight, I think thats why my back has been hurting because mine is so low i feel like my butt crack is where it hurts! lol! And mine hurts really deep

I've tried a hot water bottle as a compress before bed and that's helped before. I sometimes wake up on my back, but that usually doesn't cause me discomfort, if anything I feel better sometimes. Every womans body is different though. Sometimes if I lie flat on the ground it feels so good because all the pressure is taken off from standing and sitting. But that position gets uncomfortable if I stay in it too long.
Good luck easing the pain dear

It comes with the territory I suppose!