Is it hyperthermia? ?

Well, the heat dilates blood vessels which lowers blood pressure, and some people are more likely to experience your symptoms than others. i have low blood pressure....not low enough to require treatment, but low enough that I am prone to the symptoms you describe when I take a hot bath or in very hot weather. Check your blood pressure.....if it runs low, then that is probably your answer. Low blood pressure is good.....it keeps your arteries healthy!!!!!!!