I've been getting rashes and itching now for about 9 months and still haven't figures out what is causing it. I suspect it's something I'm ingesting, but I've stopped all vitamins and have tried eliminating obvious foods (wheat, fruit, nuts, etc), but with no luck.
We had a friend who was getting rashes horribly bad. He thought it was something he was eating. You may be getting impatient on eliminating foods. He ate only foods made out of chicken or rice and bought only from Whole Foods. He did that for 6 months and had no more rashes. He slowly added back an item at a time to see if he reacted. It turned out he was allergic to tomatoes. So many foods are made using them.
When I gave up gluten, my stomach issues resolved within days. However, it took 2-3 months to see any improvement with migraines. I still have them but only about 1/2 as many.