SI pain was my very first symptom almost 9 years ago now. It started when I was pregnant with my first child, it would shoot up my spine and down my leg... my leg (left, mostly) would actually just fall out from under me.. I wouldn't feel it.
It has been a pretty frequent issue for me ever since. I was told I had spondylitis..and DDD, and alignment issues. Been through round after round of PT, to no avail.. it always comes back. I actually fall asleep every night with a heating pad (has a timer and shuts off after 1 hour), and it is a life saver for me!
I am a firm believer that Auto immune disorders are a symptom of a fallen world,, I wonder what a perfect body with no pain would feel like???
Sorry you are getting even more issues to deal with,
"You do not have a soul. You are a soul. You have a body." C.S. Lewis