n = 1, but I had pretty much this exact same issue and finally figured out that it was happening only after I slept in a position that would put one side of my lower back in a stretched/twisted position for too long (Usually on my left side and with my torso twisted a little towards prone). I started being more careful about my sleep posture and added in some suitcase walks as a deadlift warmup and haven't had problems since.
This might all be broscience but it couldn't hurt to start documenting your sleep posture and how your back felt when getting up, check it against your workout/injury log, and maybe see if any patterns emerge with the pain.