if the pain goes away i would say its detox. you could just be feeling a stiff back.
add two glasses of water to what you are drinking and see how you feel.
the swf makes me feel like what you describe.i totally skipped it. its for additional internal bathing anyway. as long as you are having a BM during the day thats good.
when fasting and detoxing old injuries and sick body parts can act up too. its part of the healing.