Heat exhaustion means you’ve probably been trying to sweat and thus lost minerals. If you drink too much normal water, you can dilute the minerals even more and thus aggravate a condition known as: Hyponatremia (salt insufficiency) en.wikipedia.org/wiki/Hyponatremia#True_hyponatre…

The thing with hyponatremia is that it’s very similar to heat stroke in terms of symptoms, but drinking water makes it worse. See: Dehydration, Heat Stroke, or Hyponatremia? The Recognition, Treatment, and Prevention of Hyponatremia Caused by High Exercise Outdoor Activities. eric.ed.gov/?id=ED443633

So figuring it out requires knowing some context, what went on before. Or you can hydrate slowly with an ORS like the ones mentioned around here (ending with -lyte) or make your own ORS. Or you can eat some stuff, get some salty nuts, eat some fruits, as a way to stock up when the day looks risky.

I’m not certain, but I think I did get hyponatremia once while hiking in the mountains in summer heat. I drank a lot of spring water in a day, like… 10-20% of my body weight and was still thirsty, and didn’t even really need to urinate, since I was losing so much water through sweat. It was very exhausting. I only felt better at the end of the day when I downed a large bottle of cheap green soda. And later, at home, when I downed drank a lot of cold soup. It just felt necessary, so I’m just figuring it out in hindsight.