I have had the same problem. I believe it to be symptoms of candida overgrowth caused by disprupting the natural flora of the gut. Looking at the other symptoms, I think I have had it in a mild form for a long time.
Anyway, I am taking antifungals, brobiotics, lots of garlic and doing a Colon Cleanse as well as restricting Sugar and wheat. The rash has almost gone now after a week and I am (after a couiple of days of die-off, when I felt horrible) starting to feel better and clearer headed than I have in ages.
Your problem may not be the same, but it could be worth a look.