What is in diet pepsi that would cause skin rash?

I have got a severe rash after drinking colas, I believe it to be that sugar and/or certain substitutes are food for yeast; some 1 day after drinking I would get a severe rash on my forehead which I self-analysed as an allergic reaction to a new crop of yeast harvested by the flood of sugar/food. I treated my forehead by applying chamomile (Kamillosan) and went on a strict wheat/yeast/sugar diet to reduced the background level of yeast in my body