Hello!
To provide a little bit of background:
In the past, I’ve experienced some pain in my lower back & hip area on my left side. I think this was a case of doing too much too soon and being eager to add weight to the bar when I ran the beginner template. This has come up a few other times when I was low bar squatting heavy/frequently, and typically it seemed to happen when I squatted low bar more than 1x/week. High Bar Squatting and other lower body movements have been tolerable when I was unable to comfortably low bar squat any substantial amount of weight.
I’ve noticed in the past that my hips sit a little bit asymmetrically. My left hip bone feels like it is a bit higher or more prominent than the right side. I brought this up when I saw my physician recently, and he proceeded to have me lie down and gently tugged at my heels and seemed to be trying to line them up somehow. He then explained that the muscles connecting my rib cage and hip bone on the left side may be tight and thus could be pulling my hip bone upwards. He said that the muscle needs to be “made bigger” or stretched out to help that, and he prescribed that I do a “quadratus lumborum stretch”.
The stretching recommendation made me think of things I have learned from BBM: that pain is complex & that stretching is not a cure for pain. My understanding of stretching is that it is useful for someone who is practicing to get better at getting into specific positions and/or is trying to become more flexible (however they define flexible… kinda like how being strong is whatever you define “strong” as).
Given that, I have a few questions that I figured would well-suited for the BBM crew 
1. Does that seem like a reasonable understanding of the applications of stretching?
2. Can muscles get “tight”? If so, is that bad?
3. Is there validity to the explanation I received about my muscles being tight and that being the cause for the hip asymmetry and perhaps the pain I’ve experienced in that region in the past?
4. Would doing a quadratus lumborum stretch allow for the muscles connecting the rib cage and hip bone to “un-tighten”?
Fwiw: I don’t ever do any formal stretching. I just lift weights and do cardio.
Thank you!