Hi
Welcome to the forum!
Low back pain can be due to muscle pull/strain. However there are other causes too. Pressure on the lumbar nerves can cause pain due to spondylotic changes in vertebrae, degenerative bone disease, herniated disc, bone disease, cancer or TB of the vertebra or bad posture. Kidney problems, stones in the urinary tract and colitis or IBS are the other possibilities. It is difficult to comment beyond this at this stage.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
Not sure, but if you suspect your kidneys, and haven't already, I'd go in and get that ruled out. You don't want to mess around.