Try supplementing some electrolytes. Cheapest and easiest way: Get some table salt and some potassium-chloride salt substitute, like the brand “No-Salt.” Mix about 1/4 tsp of each into a pint of water. Sip, don’t chug.
Also, take a magnesium supplement (tablets are cheap — buy them wherever you buy vitamins).
If you were doing longer fasts, like more than 24 hours, you’d need to do the drink multiple times a day. On 16:8, once is probably enough, because you’ll also be getting electrolytes from your food.
As another commenter noted, this could also be a sign of iron deficiency. You might want to start taking a multivitamin with iron — preferably with a meal. Your body will absorb it better with food. (Also, it can cause nausea if taken on an empty stomach.)