I don't have MS, but I have never perspired freely, which makes me prone to heat exhaustion. When I was young, I had to limit the time that I could play tennis or basketball, because I would overheat, breaking out in prickly heat, then getting chills and my body turning bright red, especially my face. And yes, I usually got a nice headache to go with it.
My dearly departed grandmother, who was born and raised in England, was a great one for quotes/sayings.