Don't forget electrolytes while fasting / in ketosis (all three: sodium, magnesium, and potassium).
A common phenomenon called keto flu. Especially pronounced the first time.
I suggest high electrolyte food, such as a nice salty soup. Seems to help.
I like to do lazy keto (which is easy, just forget to buy snacks after you run out) but always find I'm thirsty and specifically always want miso soup. Maybe that's why.
Just going calorie negative did this to me. I still eat carbs and it doesn't seem to help.
Totally normal the first days even weeks are a bit fuzzy. Your body is adapting to a completely different metabolic system. It’s very stressful.