Why do i always feel really weak/tired to the verge of fainting/passing out when i sweat or exposed to heat? happens in as little as 5-10mins in sun..

History of ?: Have you ever had Heat Exhaustion or Heat Stroke? People with that history have a lifelong higher sensitivity to get it again. They can get either condition with much less heat exposure. Could this be possible with you? I would also make sure you have a normally functioning Thyroid. Also, some medicines can cause a heat sensitivity. Also, with advancing age, people have less tolerance for heat.