Late periods?!

So I haven't had my period in 73 days and I don't know why. This happened to me before but it was like 10 months so I went to the doctor and she told me to eat more and I have been but it's still not Helping?! I'm 15 so I know it should be irregular but not this bad. And there is NO WAY I'm pregnant. Any ideas of what could be wrong with me?! By the way I'm 5'3 and weigh 115 normally. I think this is an average weight. So I don't think that's the problem. Thanks for the comments.