I saw the doctor today. She is a sports medicine/physical medicine & rehabilitation doc, and seemed quite knowledgeable. She ordered x-rays to rule out a fracture of the sacrum (which I was pretty sure I didn't have), and the x-rays revealed something interesting. No, I do not have a fracture, but I do have L5 (lumbar vertebrae #5) - S1 (sacral vertebrae #1) assimilation, also known as lumbosacral transitional vertebrae, on my left side only. Basically, this means that on my left side, the lowest lumbar vertebra has an extra attachment to the sacrum, which means that I have less flexibility through the SI joint on my left side. The picture below illustrates my condition quite nicely.
Fortunately, it is a rather benign condition, UNLESS you happen to have what my doctor called "the perfect storm", wherein I was 1) recovering from posterior tibialis tendonitis, 2) slipped on porch steps, 3) landed on my left lower back right about L5/S1. Brilliant. She said that because of my L5 sacralization, I probably transferred more force from the landing to the right side of my pelvis, stretching out my piriformis too much, and voila! Injury. That's her guess, and the PT's guess without even seeing the x-ray or knowing of my L5 sacralization. I have a good PT.
So what's the bad news, you ask? Well, the doctor says there isn't anything else she can do medically to make this heal any faster. Nada. Zip. Zero. In addition, she estimates that it will be 3-4 months from the time that I fell until I can actually train again or even think about it. Meanwhile, I need to do my PT and continue with the exercises, and get my cardio from something that doesn't hurt my piriformis.
For those so inclined, here is an article that has more information about LSTV and its clinical consequences. It is pretty benign, with possibly an increased risk in disc issues, but otherwise it is generally not a big deal.
So now I need to make more long-term plans for cardio for the next few months. I'll figure something out.