Seconding the electrolytes, but also have you fasted before the 24hr like 18:6 or 20:4? When I first started fasting I was nauseous most of the day bc my body wasn’t used to it and wanted food. After a couple consecutive tries, I got used to it. A lot of people get nauseous when their stomach is empty, so fasting can cause that until you get used to it!
If you haven’t yet, I would start with 18:6 (fast 18 hours, eat 6), and then move up to 20:4, and then up to 24 hours. Taking it slow and steady can help a lot of people. It’s what really helped me when I first started a few years ago.
If I drink too quickly (especially cold and/or carbonated beverages) on an empty stomach, it’ll seize up and hurt acutely for a minute or two. I also get queasy if I drink too much at a time. Both eventually subside, but are uncomfortable in the moment. Personally, I have to be gentle with it.