I’m going to presume in this case that we are dealing with pain in the lumbar area, which often manifests more on one side than the other, or may switch from side to side or can be felt diffusely all across the low back and into the buttock and hip area.
If you have suddenly become aware of numbness between the legs (The saddle area} or a dramatic change in bowel movement or bladder function. You should seek urgent medical attention.
Also I am presuming that serious pathology has been ruled out such as cancer or osteoporosis etc. and that you are in reasonably good health otherwise.
Swelling and Inflammation
The underlying cause will be inflammation and swelling of the soft tissues in the area. It will be a matter of degree from slight swelling and nerve root irritation up to herniation of the vertebral disc’