Did some research on that after you first posted about it
When my back went years ago the physio diagnosed torn muscles in my back. Next physio diagnosed a slipped disc. Had a bad pain also from the inside of my groin down to my knee that I thought I’d done when I slipped on some ice. Strange thing was, whenever my back went, the pain always felt like it was in my pelvis rather than my back and I had to walk round the house clinging on to the tops of doors, door frames, worktops, etc to take the weight off my pelvis rather than my back
Self-diagnosed now that I’ve got a problem with the psoas muscle. Been doing some yoga-type stretches that I downloaded to stretch the muscle. Blimey. Big difference. You’re right