An excellent summary of the condition can be found at The Chronic Candidiasis Syndrome page. Food allergies often cause stomach discomfort and fatigue and may be difficult to track down on your own. Wheat and dairy are easily the two most common food allergies and should be avoided. Studies have linked wheat and dairy allergies to skin rashes and asthma. This was surprising to me, but I could repeat it by eating wheat one week and not the next.
Food allergies occur when the immune system mistakes foods for invaders causing an immune reaction.
The two major types of immune reactions here are IgE and IgG. IgE reactions cause symptoms anywhere between a few minutes and two hours after eating the food. This generally includes stomachache, gas, etc., and may also cause fatigue and mental fogginess. Food allergies can trigger migraines and migraine equivalents. IgG reactions are generally delayed reactions occurring anywhere from two to twenty-four hours after consuming the food. These reactions are often in the form of fatigue and fogginess.