When you come out of the swimming pool on a hot day, water on your body that evaporates at a faster rate bring more thermal energy away from the body. Water changes state, when there is a change in state in water from liquid to gas,it requires a lot of thermal energy to be removed from the body. On a hotter day, the rate of evaporation is faster thus you lost thermal energy in a shorter time so you shiver.
Why do people shiver when feel cold?
The human body needs to stay at the same temperature to survive - about 37 degree Celsius. When you feel cold, tiny sensors in your skin send messages to your brain telling you need to warm up. Your brain sends messages to nerves all over your body telling your muscles to tighten and loosen really fast, which is what we call shivering. It does this because when muscles move they generate heat.