That lower back pain tends to mean the kidneys are not flushing well... drink drink drink!!!
On day 18 of my 60 day megafast I passed a small kidney calcification, and fasted through with the support of my excellent naturopath. I took homeopathic and herbal support for both my kidneys and later my bladder (owie), and was fine as frog's hair in a week.
Anytime I get lower back twinges while fasting I seriously step up my water consumption! If it continues I drink fresh parsley tea, or make tea as recommended by Edgar Cayce for cleansing the kidneys: Steep 1 T. chopped fresh Watermelon seeds in 2 cups boiling water til cool and drink freely.
Heat helps relieve the pain, and I believe a good soaking bath helps us to hydrate.