In the last month or so, I have been waking up with my kidney area painfully stiff. This is not the same kind of pain I might get after straining my back during a day of heavy lifting. It feels like I've spent the night bent over backwards, as if I am on my belly but with several pillows under my head. (I only sleep with one pillow.) The pain goes away quickly enough - after just a half hour or so as I go about my day. Itr's not a deep pulle dmuscle kind of pain, though it is still quite difficult to move for the morning. When I was young I slept on my back. When I got married, the snoring became a problem and I got used to sleeping on my side/belly. I've lately swtiched back to sleeping on my back as that seems to go a long way toward waking up without pain. The one thing that's changed in the last few months is that I've gained ten pounds, which accumulates around my belly. It's no surprise that gaining weight would cause back pain, I'm just trying to figure out why my back would get stiff when I'm sleeping on my belly.