Usually your SI joint doesn't get much action. In most men the joint fuses by the time they reach adulthood. Women need flexibility in that joint for childbirth, as the sacrum moves in the joint, the pelvis widens to make room for the baby to get out. It's designed to only to slide up and down in the pelvis. However, it is possible to torque it and pull one side of the sacrum to the side, up, or otherwise grinding into the pelvic bone in a generally unpleasant way. This is sacroilitis or SI injury.

The past week or two I have been dealing with some pretty major discomfort. I first felt it around my right shoulderblade. It seemed like no matter what I did in my yoga practice, that area was never quite right. Shortly after I noticed weirdness in my alignment, I got the tell-tale quarter-sized pain on my back. If you have ever whacked out your SI you will know exactly what I'm talking about. It's that place on your back, a few inches below your waist, where there are a pair of dimples. That's where your sacrum hooks into your pelvis, and that dimple is the hurty place. In my case, it was the right "dimple" or "button" as I like to call it where the pain was sharpest, but as I mentioned, I had referred pain all the way up to my shoulder as well as all around my right hip and thigh.

I really am not sure when or how my SI pulled out of alignment. It's an extremely common yoga injury, but it's just as likely that I did it while I was dancing, sitting strangely, or even sleeping. Oh the joys of hyper-flexibility!

Fortunately, last night I was able to get into a yoga class with my instructor who is specially trained in therapeutics and she was able to set it back. She used Gillet's Test to identify how it was misaligned and then got me on the mat and gave me an adjustment to put it back in place. I was able to make it through class relatively pain free. The "button" is still a bit sore, and the muscles deep in my hip are tight and achy, but that will get resolved in my yoga practice. I had been walking around feeling like one leg was longer and my spine was crooked and that went away as soon as my instructor made the adjustment. The relief was amazing!

Moral of the story: be kind to your SI!

Resources:
Sacroiliac entry in Wikipedia
"Out of Joint" by Judith Lasater

ETA:
Create Stability in the SI at YogaNerd
Low Back Pain Management Program (PDF)