This sounds almost exactly like my problem. The pain always grew as I slept. At worst I awoke after 4h of sleeping every night for 6 months before I knew what was going on. Countless clinics couldn't figure out the problem. Stretching was the key to finally relieve the pain like in the picture you posted. Like most people, when the problem is resolved and everything is normal I stopped stretching and the problem started coming back (Why are we like this?). And thus I found my way here. Thank you for being so detailed. I will try that stretching technique as I work on getting rid of the darn Quadratus Lumborum pain once again.