I had terrible back pain, too. I could barely walk and used a wheelchair to get to my appointments. So my oncologist got me oxycodone and sent me for an MRI. The oxycodone messed with my head and did not help my pain either. I saw the orthopedist surgeon next and he said I was having micro fractures back there. It could just be because of my age or the cancer. So he did a sacral bilateral surgery, inserting three screws to support the area. It was a piece of cake. And I saw a palliative doctor who changed my pain med to hydromorphone. Oh my gosh. A world of difference. I walk normally, although slow to start. And the pain is way, way down. Maybe this can help you. I only have two weeks left of the immunology treatment. Bone marrow transplant next month. So I am wishing you the best. Good luck.
You need to be diagnosed with an MRI, CT scan or PET scan, then treatment, and given appropriate pain meds. A Fentanyl patch can be helpful. Discuss it with your doctor.
^MRI
Have you seen a NRI or Xray of your back. Your spine may be weak and you could have a fractured vertebrae. Hope not but the vertebrae where weak bones show up first. I had 7 fractures before we finally got it stopped with surgery on 3 and treatment. My heart goes out to you because I know the pain you have to contend with.