I have this problem right now. I have a constant pain in the attachment of my spinal erectors on my right side.
I had a physio look at it and she said my glutes weren't firing correctly, so I've been doing exercises to fix that. Here's a really good start for activating the posterior chain (well, it helped me, anyway). I do this every morning when I wake up and the pain is about 25% of what it was... if this keeps up then hopefully I'll be pain-free in the next week or so.
BTW these exercises look easy but they're really hard.
https://www.youtube.com/watch?v=4BOTvaRaDjI