Cognitive impairment onset following gastroenteritis. no substance abuse, depresssion, anxiety or medication. normal bp, bloodsugar and crp. ?

Sudden? Gradual?: You don't mention how or if the impairment was confirmed, but distress, especially chronic and untreated, can itself take a functional toll on mental activity quality. If you can find even symptomatic relief for the GI pain, you might discover a cognitive improvement too. If not - maybe a psycho-neurological exam can find a basis. Nutrition also may play a role. If severe, seek answers. Good luck!