Lilac-Sophia,
was this back-pain/ache apparent BEFORE you began the fast?
If not..........
"Crises developing during the fast are not different from those developing at other times and are not to be cared for any differently. They are all Orthopathic in character and, although often disagreeable, should be welcomed............
......................The increase in symptoms previously noted should be considered as critical action. Fever, dizziness, headache, vomiting, nausea, BACKACHE, skin eruptions of various kinds, but particularly urticaria (nettle rash), jaundice, diarrhea, etc., may develop".
Shelton......... http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch28.htm