Most pain in the lower back during a fast is caused by the colon (intestines) dumping toxins--you can endure.. or you can try to take an enema to relieve some of the pressure by eliminating both toxins and fecal matter (your last meal before starting the fast is still in there)
You can also drink a tea like SMOOTH MOVE to help with the pain--- this is a detox symptom and can be an indication that your last meal was pretty toxic (as in a greasy hamburger or meat dish or full of processed or additive ingredients) OR that you have fasted so much you are now detoxing at a much deeper level from years ago.
Which ever the reason, drink LOTS of water to help flush the toxins and I normally drink 2 cups of the tea to provide some relief. sorry this is coming 2 years later, but hope if you fast in the future, this helps.