I have the same problem with the pain in my back, but, it moves around. On the back, it's only one spot at a time, but it can be quite intense. I've had diagnostic tests and they haven't found anything. It isn't the same kind of pain as when all the organs (brain, lungs, heart, liver, everything) seem to be 'squeezing' out toxins during the fast, and hurt (that was my experience last time I did this fast.) I don't know if you'll buy my explanation, but this is what I attribute my pain to, based on my knowledge of the subtle & causal body, life cycles and the way energy works. I feel that the pains in the back come from two potential causes. 1)That you were mortally wounded in this location in a past life, and 2)That it is the location where those who try to take energy from you are fastening themselves energetically. These are people who you might have an emotional connection with, good or bad. And it may not be malevolent, they may just be needy. I do meditations featuring gold light to dissolve the pain, but it often recurs. I can tell you some if you're interested, but this may be a little too much for you. Hope it helps, anyway.