<<Disclaimer: Verify this information before applying it to your situation.>>
 
I have often had the problem with "mental fogs" in the past.    On every
occassion I found, after checking, that I had been getting a hidden grain
of some sort (I react to ALL grains, including rice and corn).
 
The mental fog is generally my earliest warning sign that something is
wrong.     The same type of problem that is so easy that you can usually do
it in your sleep suddenly becomes impossible -  the pieces just won't fit
together - or they seem to fit when actually they don't at all.    Later,
when you're normal, you look at it again and just cannot imagine how you
could have had trouble with anything so simple.
 
The key is to recognize that when you have trouble doing something that you
can ordinarily do without any trouble (and it is not an unusually complex
example) - this is a warning sign that something is amiss.   In my case
it's always been a warning that somewhere a grain is slipping thru.