Boy, I can relate. On day 18 of my 60 day mega fast I passed a small kidney calcification--ouch! Luckily I was supervised by my wonderful naturopath and was able to fast through with homeopathic/herbal support and everything came out just fine. But I had a rough week there.
I am always very conscious of my kidneys while fasting, and work hard to keep my fluid intake up to flush them well.
I wonder if the rash doesn't indicate a congested liver? All the cleansing and fasting I did never affected my skin troubles ("adult" Acne and "rosacea") until I finally flushed my liver clean. Amazing, cheap and effective protocol.