Lay on your back on the floor. Bend your knees and then press the small of your back to the floor. Hold it for a few minutes. Also, spend some time on your back on the floor and pull your knees to your chest and hold them there for a few minutes.
These exercises have helped me when my back was flaring with pain.
I'm so sorry to hear of your terrible back pain. I know how it is. My back was very bad last year for about 6 months. It is much better now, but I have to be careful. I had physical therapy and hydrocodone to get me through it.