I had been regularly eating yoghurt, or kefir, and I've taken probiotics capsules.
I stumbled upon an article. It describes the results of a study that found the following: people taking probiotics experienced digestive issues and mental sluggishness.
One of the things that appeared to be causing my mental sluggishness (during the last few months) was probiotics.
The article:
Probiotic use is a link between brain fogginess, severe bloating
Date: August 6, 2018. Source: Medical College of Georgia at Augusta University