Re: Periformis ??
Nice to see you again, but sorry you are experiencing a "new" pain.
I am merely guessing here, but this sounds like the type of "compensation" injury that often happens with the spine.
Just like the worry of "adjacent disc disease" or the "domino effect" when one is fused, when only one SI joint is fused and therefore has no movement, it makes sense that the other SI joint has double the stress on it.
It may be that when you bent over, you may have twisted very slightly at the same time, or some part of the movement caused extra stress on that left side because the right side couldn't move.
It is really common to have piriformis issues when there are problems with the lower lumbar area. After I was fused from L3 to S1, I had pain that I primarily felt in my left "hip", but often in what I thought of as that left "quadrant." The piriformis muscle runs from the area of sacral spine over to the attachment near the hip. When it gets tight, the resulting pain often radiates from the buttocks, down the thigh, and up into the spine. Piriformis syndrome is often confused for a herniated disc, sciatica, or another back issue.
What I wasn't clear on in your post is whether the pain is on the front or back side of your thigh??