What Causes Teeth To Chatter When Out In Cold?

yes I would like to know what makes me get so cold that I freeze I mean really freeze!!!!! my teeth even chatter!!!! I have to cover my head and get under my electric throw to get warm or take a hot shower! It makes me really sleepy too even if I've slept for 8 hours this is so terrible that I don't like to go outside cause the cold weather make's it worse!!!! what does this mean?????