Congratulations, you’re in ketosis! You’ll probably feel great and full of energy for the rest of your fast. Keep up the electrolytes, and remember hunger comes in waves; if you feel a craving, it’ll pass. And if you feel faint, take a knee. That’s your body wanting to increase the blood pressure to your brain, and if you leave it no choice, it’ll lay you out flat on the floor to do it.
Yeah this sounds like fat adaptation. For some people, they’ve lived on carbs as their main source of energy their whole lives before recently trying out fasting, so the body has to adapt to ketosis at first, even down to the mitochondria changing to better feed on ketones. This would be why it can be hard to fast a few days at first, and why it sounds impossible and insufferable to people who never have before. Once this shift is made we run better and better on fat (ketosis).
The other thing is our bodies become more efficient at storing vitamins and minerals when we do eat afterwards, especially as insulin resistance improves, to prepare for fasts. Our liver can store over a month of vitamins from a healthy diet. One more reason to keep our livers healthy. :)