I’ve noticed the same thing on a couple of hot trips, it just seems to knock me out more than it used to. What’s helped me is carrying one of those little handheld fans with a spray bottle, sounds a bit daft but it really takes the edge off when you’re walking around. I also keep a thin cotton bandana in my bag, soak it in cold water and wrap it round my neck, it cools you down fast. I try to stick to shaded streets or duck into a café for a quick sit down rather than powering through in the sun, and swapping fizzy drinks for coconut water or even just plain milk has worked better for me when I start to feel wiped out.