The author went through keto flu, which is a series uncomfortable adjustments your body makes as it transitions from glucose fuel to ketone fuel (ketosis). People who begin ketogenic diets know this well. Fasting feels very similar. The most acute symptom is often "brain fog", which manifests exactly how the author described.

Ketosis takes several days to begin while the body burns through its reserves of glycogen, at which point the symptoms begin, then lasts several days while basic adaptation takes place. Full adaptation to match one's athletic performance while glycogen fueled can take months.

The author didn't even mention the words ketosis nor ketones, so I have to imagine she went into this quite blindly. Once you've gone in and out of ketosis several times, you don't experience flu symptoms nearly as much and sometimes (me) not at all.

For most people a fast starts to feel magical after about day 5, where keto adaption has taken place and the mental habit and craving for food starts to subside. Obviously this will vary from person to person, so maybe the author didn't quite get far enough along to get out of the woods, so to speak.