Hi ISC member,
Before answering your question, there are some things that one should know about his/her body.
The HYPOTHALAMUS is a part of our brain which is just above the "PITUITARY GLAND".
It is often referred to as the body's "THERMOSTAT". The hypothalamus senses the temperature of the blood in our body and then compares it with a "fixed" body temperature something like a set point which is the desired temperature. If the actual temperature deviates from this set point, it automatically adjusts the temperature.
The best example would be "the new A/C's that has come out with sensors which adjusts the room temperature at an optimum level" automatically. Now the hypothalamus sends out signals that cause one to "SWEAT" if he/she is too hot or to "SHIVER" if he/she is too cold.
"SWEATING COOLS THE BODY". The sweating glands in our skin secretes the sweat when it receives the signal from the hypothalamus and as a result the body cools down because as the sweat evaporates.
"SHIVERING WARMS THE BODY", because when the muscles in our body start to shiver, heat is liberated due to friction between muscle fibers. In addition, it makes us feel uncomfortable, which prompts us to seek out for a heat or cold source. If your body temperature is too low, you'll probably look for a warmer place under a blanket. If it's too hot, you'll look for a cooler area.
When you are running a fever, the body sends signals to the hypothalamus telling it to raise the set point of the body to a higher temperature. Often, but not always, this may be a response to an infection in our body, such as a bacterium or a virus (the virus which is actually the cause of fever). The immune (resistant) cells respond to the infection by secreting molecules called as "pyrogens".
The pyrogens will travel to the hypothalamus via blood, where they re-set the body set point (increase). So now, the hypothalamus tells the body that it is cold than usual. Remember that the temperature may be normal in this condition; but the temperature is lower than the set point, which has to been raised immediately.
"SO YOU WILL FEEL COLD AND TEND TO GET UNDER A BLANKET AND SHIVER".
(hope my answer cleared your doubts)