Way back in my 20s {30yrs ago} I pulled something and the dr called it a sacroiliac strain.
After it healed I had no problems in that area until a few years after my RSI/TOS came about.

Mine is a ache that can occur deep inside the low back/hip area, and a ligament/muscle thing it seems like to me. Mine is only on the left side which is the side that was injured all those yrs ago.
Mine comes and goes depending on activities.
Takes usually a week to calm it down.

plus my hips are tilted a bit as well as ribcage & shoulders
So my chiro always has something he can work on LOL.

Hope you can keep getting continued relief- if not afix.