indeed heat and humidity can be the duo that makes you dizzy. being well hydrated is important but you're forgetting about all of that sweat you've created. you're losing valuable electrolytes. in the future i'd make sure to have one of the sports drinks on hand that has sodium and potassium mix in it. you can buy as powders to add to water or in bottles. there are lots of options out there. often i just fill up my water bottle with 1/2 water and 1/2 electrolyte mix. the simple rule is that when summer comes around don't forget your electrolytes! clearly if you've taken precautions and you still end up with problems you need to seek medical attention.
could high humidity cause brief dizziness during a brisk walk ?