Last summer my lower back started locking up when I bent forward. I hurt for a week or 2 then it would calm down. Throughout the summer into the fall and winter I'd get this locking even with simple forward movements. My right knee and foot also hurt so I consulted an ortho, podiatrist and physiatrist. No one could explain why my lower back was locking like it was. I also had pain in the area of my piriformis. An MRI only showed some arthritis in my hip and right SI joint, 4 months of PT didn't help and the exercises made me hurt worse. I too got so I could only sleep on my side with a pillow or on my back with a pillow under my knees.

This spring I saw a different PT for a bike fit, instead he looked at me. He discovered my left SI joint was locked which he loosened, gave me a couple of exercises and I've been pain free ever since. No more foot (I can walk barefoot again) or knee pain, no locking of the lower back. I sleep comfortably with no pillows. No pain on the bike.

I'm still holding my breath afraid this will all come back but I've been pain free all summer.

I did have my fitter check my bike fit. He suggested changing my saddle because it was too much of a Y and wouldn't allow my hips to form over the saddle and I was riding forward on the nose. I don't think the saddle caused the issue but the new one is more T shaped, is a better fit and gives me more power on the bike.

My 1st PT suggested I try Pilates but it made things worse.

If you haven't had a bike fit do so, however, I suspect there is something going on with your body causing the pain. I never realized how debilitating back issues can be until this started. It may take some time to determine what is causing it but keep searching until you find an answer.