Your enzymes starts to be inactive as they lack enough heat. If heat is too high they denature. The enzymes are important for many functions as they catalyze processes in the body. For example when you get fever doctors try to keep it low er as it may denature your enzymes. If your body temperature is too low, below normal temperature, many processes do not work. If you are asking about the causes of it perhaps the surrounding temperature is too cold, perhaps the liver is not working well, perhaps its a vampire?
Definitely try to raise your temp. Bad circulation can lead to colder body parts. To raise your temp cover your body in blankets, drink and eat hot things. Going outside when its hot can help. If your inside turn up the heat. Lower body heat is a big deal, and you need to go to the doctors as soon as you can.