Hello, My highest suspicion is still the heat. Even if you stay well hydrated it is still possible to develop heat exhaustion (layman s term is a sunstroke or heat stroke). Essentially what that simply is, is the body fails to regulate body temperature and it rises due to the heat outside. As we age, the ability of our body to regulate its internal temperature weakness and therefore environmental stress such as heat or cold can have a greater effect. So what happens is that you body temperature rises during the day due to the heat, and then what happens is that you essentially do have a fever, but not because your body raised the temperature, but because your body was unable to keep cool. The symptoms feel exactly the same, and you feel cold. The as your body temperature comes down you can have profuse sweating as well. Headaches are very common with heat exhaustion. The best advice I can give you is to slow down a bit, take frequent breaks to cool off and don t work too hard in hot weather. And as you have already noted, staying hydrated is also very important, but sometimes that simply isn t enough. I hope this helps. Sincerely, Kristoffer Nissinen