It is also possible that the blood or some other fluid under the skin is thickening during the hot weather upright prolonged exercise
and I don’t know if this is normal during exercise
. I do feel that my blood is thickening as after a golf round as I often get a a bit of a headache that evening. (However, it is not related to insufficient fluid intake).
I will try some ibuprofen, as it is does thin the blood, before or during the golf round and see if that helps.