Why does a person's face become flushed when they are working hard?
It is your body's way to keep itself from getting overheated.
Lots of things happen in the body when muscles are exerting themselves. First of all, muscles that are active must burn more energy in order to maintain the work. Sugars are metabolized in the mitochondria; when this happens, much of the energy released from the sugar is in the form of heat.
The heat then increases the temperature of the tissues, which increases blood flow to the tissue. Doing so warms up the blood.
The warmed blood circulates through the body, which travels to the hypothalamus (tucked away in the middle of the brain). Because its job is to maintain homeostasis, the hypothalamus responds to this increase of temperature, and tries to correct it. One of the most effective methods of releasing heat is through the skin.
The hypothalamus sends a hormonal message that causes the skin's capillaries to dilate. This increases blood flow, causing the flushing. It also causes an increase of sweating, which further reduces body temperature.