What Causes Absent Periods For A Long Time?

Im 16 and I have missed my period for 3 months now, I'm not pregnant and the only things I have changed is a new diet and exercise plan. I have been menstrating for about two years but last year during the school year I didn't have my period for 6 months straight..... I'm confused and my other doctor isn't any help. Is it normal or is something wrong with me?