I also have the same back pain after my 5 day of fast even though i did enema every day. i am doing a grape/lemon fast.
What i just did is doing me good is using black peper with a little bit of water (so that it sticks to skin) and vigourously rubbing it on the pain area. (cayenne peper would work even better i think ) .
I give a nice hot feeling...
almost like a patch. very relaxing...just another thing to add to the list of to do's ; )