Why do I have hot flushes?

I've been having hot flushes on and off for two years.I do not have pimples on my face so it's not rosacea. My face would suddenly turn bright red in even moderate weather. I'm 16 so it's not menopause and I doubt it's dehydration. I'm 5'4 and about 105 pounds. I exercise almost everyday so what it causing this? It only happens for a few minutes then it goes away.