Yes, food can cause fatigue and brain fog.  I believe the food triggers histamine release which causes the symptoms, but someone will correct me if that's too simplistic an explanation.

Can't remember if you've tried Allegra, Xyzal or Zyrtec.  Those are less sedating than hydroxyzine, although hydroxy. might be better for brain fog in some people.  I looked it up, and 50-100 mg. of hydroxyzine per day is a normal adult dosage.  You might want to ask your doctor if you can add one of the other antihistamines or increase the hydroxy.  In addition, some MC patients take 150-300 mg. ranitidine, twice a day.  Doxepin also can be sedating, so it's important for your doctor to adjust your dosages to help you feel better rather than trying anything on your own.

Have you looked at the low histamine diet yet?  You can try it for a few weeks and see if it helps any of your symptoms.  The one I prefer is at:

www.urticaria.thunderworksinc.com/pages/lowhistamine.htm