The most likely response is that you are beginning to feel the impacts of dehydration. Normally, the typical person is not 100 percent hydrated. Include exercise and a warm climate, and it spells dehydration in a big way.
Do not depend on thirst as a signal to avoid dehydration!
Your body's drive to consume is not nearly as effective as its drive to consume, and the thirst system is even less effective throughout theexercise. Therefore, you must prepare to consume early and often.