I have the same problem as well. If you look up Candida it's one of the side effects, as well as fatigue. I notice when I am on the Candida diet that those side effects improve, but I have never disciplined myself to stick to the diet and see it through to the end.
Yes it is called brain fog, your ability to think is gone
I've had it for years (and years). I really feel (my own situation will attest to this) that it's a reaction by the nervous system to anxiety or flat out fear. I get waves of it, particularly released from my feet that even I am allergic to. Every minute of my life now is consumed with this crap. Debilitating? That would only begin to describe it. I do know that, in my case, smokes, caffeine, any stimulants make it much worse. If I have a hangover, and I've had coffee and a few smokes. I could walk in to a gym and some would start sneezing. My Dr. doesn't know what I'm talking about.