could be temperature regulation:
http://en.allexperts.com/q/Weightlif...ercising-1.htm

Overheating, or hyperthermia, can be a problem with strenuous exercise or when working out in hot weather. Overheating can cause mild to life-threatening conditions. Heat exhaustion, a moderate form of hyperthermia, is characterized by the following symptoms:

* Lightheadedness, nausea, headache, hyperventilation, fatigue, and loss of concentration.
* A high temperature (above 103 F), possibly accompanied by complaints of chills and clammy skin.

http://are.berkeley.edu/heat/humidtxt.html