Hi kandice, google 'left flank pain' could be kidneys, but may be not. a lot of possibilities on that side. fasting is a huge ph change to the kidneys. since its gone on for two days now i'd get thee to a doc just in case. might just be a back muscle strain, had that once, both sides. thought it was the kidneys too, so my doc sent a urine sample to pathology but it wasn't. took 10 days for the muscle strain to heal completely. maybe you stayed in one position too long. keep us updated.