I don't have osteoporosis, but I have had a lot of problems with my S-I joint (sacro-iliac - not sure of the spelling, LOL - basically, where your pelvis ties in with your spine...), and my Rheum actually did an X-ray as part of the diagnosis, because I guess S-I trouble can be associated with Lupus? I dunno....the X-rays were inconclusive one way or the other - but I have had some serious trouble and been to physical therapy several times over the last several years to deal with pain.
Sometimes it's mild pain that just eats at me - sometimes it's like a muscle strain, and sometimes it feels like somebody nailed me in the back with the edge of a shovel. One time, I was in the middle of a horse pasture and I bent over to pick up a rock about the size of an egg...something slid out of place and I could...not...MOVE. I had to sink to my knees, in excruciating pain - I was sweating and panting - I could not even take a full breath. The horses thought i was nuts, LOL....After about 15 minutes I managed to crawl an inch at a time to the fence and after another 20 minutes I could pull myself back to my feet and shuffle, bent over, to the barn and get into a chair. I popped 3 ibuprofen and just kinda sat there, trying to breathe and maybe realign stuff a little. I did eventually (with help) get tot he car and get to the doc, but it was UGLY. He said my pelvis was almost 2" out of whack that time - but he fixed it!
The good news is that the PT worked REALLY WELL, and as long as I am careful and stay strong in my core, I tend not to have problems. It meant changing some habits, though.
I dunno if that's what's going on with you at all, but I guess I'm just sharing my experience in case it's helpful. It may or may not be tied to the lupus - but man, if you think you cracked something in your spine, DON'T MESS AROUND....get it checked ASAP.
I am sorry you're hurting.... :-(