Thanks for responding.
If the reaction is that quick, and if it isn’t stress (or relax) related, the only thing I can think of is the onset of digestion.
I’m thinking along the lines of saliva production, which starts even before you start eating. Maybe the production of stomach acid and the movements of the stomach’s muscular layer also start very quickly.
I don’t know how these reactions could lead to brain fog and fatigue, though. I can only guess. Do you experience extremely low blood sugar, or other digestion-related problems?