I'm gonna chime in with a possibility, in Jager-theory style :),
The pelvis is a bone structure with the two deform-able SI joints at the back being the biggest issue here. There is one at the front too.
That nest of ligaments and whatever, can be twisted, or irritated by overused, traumatized muscles, or those in spasm - sometimes due to imbalances etc.
Particularly through the right or left side hip areas. Pain is fired in all directions - low lumbar being a typical one.
I have had this for years, negative x-rays, although I have some clunking now. But with this understanding/model of operation I have really the best back of my life right now.
I wrote a post on the trochanter belt, easy to try for a few days to see if things improve. The T-belt actually forces Left/Right symmetry in the pelvis and helps the SI joints out, while they are in recovery. It worked for me.
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