Well, like I said, I have lower back pains. Not that often, but when it happens it's always in the same situation: when I'm lying down. Mostly when I'm lying down on my stomach. When I round my back, the pain goes away, and when I round my back for enough time, I can then relax and the pain is gone. Sometimes the pain is a bit higher, like waist height. Other situation: I'm lying on a hard examination table at the PT's on my back, with my legs stretched and I feel the pain. When I bend my knees the pain very slowly goes away.
I told my PT this (I had some sessions for another problem) and she didn't seem concerned. She just showed me a pelvic exercise that was good against lower back pain according to her. She also said I was remarkably good at the exercise, seeing as I had never done it before. (flexible pelvis?). I had to lay on my back, knees bent, and "flip" my pelvis (push my lower back against the table).
I have no idea if one leg is longer than the other...
Sitting isn't really a problem. I find it uncomfortable sitting on a normal chair for a long time, but it isn't painful.
These lower back pains don't bother me that much, but if it's a sign of something worse.