I'm glad you posted! Welcome to the discussion boards!
Though I am not a Doctor, I feel that through experience, this discussion is right up my alley, unfortunately!
It sounds to me like the area you are referring to may be the SI (sacroiliac) joints. Here is a link that will show you where they are located, and it also discusses various reasons for back pain. Keep in mind that 'referred pain' is a possibility too, it's a pain that radiates to another part of the body, so that must also be considered.
As you know, pain from Fibro and Sjogren's don't show on x-rays, and the beginning of arthritis does not show up at first either, it takes time. The good thing about these x-rays is that you have a base line for comparison, something to look back at when you have these repeated, to see if joint damage is taking place, and hopefully it's not!
I understand the compensating very well, and what my Chiropractor has always told me is "We are ONE UNIT, when one thing is injured or out of place, it affects something else." So I think in this case PT is a good idea, however if you are suffering to this degree, it make us wonder if PT's IS worth it.
PT seems to be the answer of the moment for fibro, however some thoughts on this is that Fibromyalgia is a 'stress response'. My Rheum told me that my Psoriatic Arthritis most likely CAUSED my Fibro, and here is how he explained it. "If you take a lab monkey, severely injure his left hand, and do not treat it, this pain changes the brain flooding it with pain signals until every part of the monkey you touch, you get a pain response." Sorry for my reference to harming an animal, but this is how medical lab science takes place, and this is what my Rheumatologist teaches on a University level, as a Asst Professor.
My suggestion is to talk to your Doctor who sent you to PT, and possibly consider going to an arthritic swim class, they are trained to correctly move and strengthen all parts of the body in WARM water with their exercises. Your pool temp may not be ideal for you, so please ask. He may want you to attend these to get to know the exercises and then allow you to continue at home if he feels the difference in water temp in not a factor.
I hope that you can get some answers and some substantial relief, I feel for you! I hope this has helped, I'm not a medical professional, but have some major back issues.
Wishing you well,