Sounds like certain foods are triggering inflammation and thus brain fog. I had similar problems before going on a strict anti-inflammatory diet. I've been on the diet for about four years now and no longer have this problem. A few times I have tried to expand my diet to include slightly more inflammatory foods (e.g. pork) and have paid the price every time.

"Normal food" means nothing. Hamburgers are about as normal as it gets and are massively inflammatory. The same goes for fried foods, oils, bread, sugar, processed foods, etc. The vast majority of what people the world over eat these days was never intended to enter the human body. Nature did not intend you to eat hamburgers. It seems many folks with PCS have a reduced tolerance these constant dietary insults to the body and brain.