Your pulse will be rapid. You may have a headache or feel sick to your stomach. Stop working, move to a cool spot, and lie down or rest with feet slightly elevated. Drinking liquids will help. Heat exhaustion, unlike heat stress, is fairly serious and you should try to go at a slower pace for a couple of days.
Heat stroke is the most serious type of heat sickness and is life threatening.
Don't let your reaction to the heat get to this stage. Pay attention to signs of heat illness before you experience heat stroke. A doctor or an ambulance should be called when a fellow worker seems to have heat stroke. A person with heat stroke will appear confused and many show poor coordination. Move that person to a cool place at one.