I'm pretty sure I figured out what was aggravating the sacroiliac, a particular pilates move I did today for the first time since I messed up the joint last Thurs. I'm annoyed that the problem is back after it receded over the weekend (though did not abate completely). But at least I know what it is.
Part of the despair of pain is not knowing how to ameliorate it, which makes me feel out of control and helpless. My knees I can handle better because I've dealt with them for so long. I know there's no substitute for time--a chance (many chances) to test and see what helps and what hurts. However, I have less and less patience having dealt with one thing or another since age 16.