What Causes Hot Flahes And Tiredness Before Onset Of Menstruation?

I get periods of the day where I feel extremely warm (like uncomfortably warm) especially in my face/ears. It doesn't cause me to sweat though. During these times I feel opressively tired, like if I don't sleep I'm going to die. So I usually fall asleep, and sleep for about and hour or two. When I wake up most of the symptoms are gone and I just feel groggy.