I definitely remember having a sore "lower back" when detoxing - I'm not sure if I attributed it to kidneys or muscle ache, but it would make sense that the kidneys feel "challenged" as they are the major excretory organ and have a big role to play in maintaining a balanced inner environment. And the amount of toxins we take in, lordy the liver and kidneys would be working overtime. Also, if you retain water while taking pills, then they would be working to pee it all out. I would say that if you are having problems going to the toilet, or if the pain intensifies, go to the doc.
I misspelled the title... sorry! I meant Kidneys and for anyone who should ask... My "tylenol" intake is very, very low.