Middle back pain while on MC

I'm on day 7 of The Master Cleanse and have been having a pain in the left side of my back, about midway up my back. I've done the Master-Cleanse once before (for 14 days) and was plagued by the same thing. I was afraid it was an iron deficiency, so I took an iron supplement last night, that didn't help. I also drank and Odwalla high protein shake, as I figured it was pretty safe while on the lemonade fast. I am very worried that this is some sort of deficiency, because I am not totally convinced that lemon juice contains all the vitamins and minerals I need. The pain seems to be worse when I drink lots of water, and subsides between waters and lemonades. Sometimes I have these pains while not fasting, and eating normally, (lots of junk food) and the pain would go away if I ate red meat (a cheeseburger and a diet coke). Does anyone have any ideas? I've just found this message board and am excited to be doing the PB shakes, I drank my first one yesterday. I don't want to come off the Master-Cleanse for at least another 5 days, but this pain in my back makes me very nervous. Any suggestions would be greatly appreciated. Tweet