Sounds like you've developed sciatica. The muscles in your back seize up with the pain from your trapped sciatic nerve just above your pelvis. A muscle relaxant from the doc can help in the short -term. If you have an upright kitchen type chair with arms that will support your resting forearms; that will allow you to sit, and let things relax. If you've got such a thing as a pull-up bar across a doorway; hanging from it now and then may help. Oddly enough, the best release from the spasms is to walk. Counter-intuitive , I know. Use a stick at first-about shoulder height, so you can take the weight if you need to rest. Walk in an upright posture and swing your arms as much as you can. The gentle natural movements relax and realign your back. A few minutes at first, and increase the time and speed until you're walking normally. You'll be surprized how quickly you start to feel better. Good luck. Remember, lift with the legs bent, not the back!
My own sciatica over many years.