If the bone density comes back OK, then I think I'd be wanting to see an orthopaedic specialist to see what was going on with the vertebrae and discs in the spine. You could have a collapsed vertebra, or an odd curvature happening there. Perhaps your GP could organise a spine xray or mri to check out possibilities?
If the bone density comes back low, then get a coeliac (celiac) screen done as one of the biggest causes of low bone density is undiagnosed coeliac disease (because of the malabsorption problems associated with it). Coeliac is another autoimmune disorder and runs in the same families as lupus, etc.