Haven't had a period in 6 months??

So recently I have been very concerned due to my lack of period. Initially I believed it was normal, that skipped periods weren't abnormal and maybe it was just a change of my cycle. I even have a friend who (it is natural and her doctor found it being normal for her) only has her period every 3 months on average. But as the months passed by I started growing more and more concerned, feeling it wasn't normal for a teenager to have missed it for this long. And to avoid the obvious question- no I am not pregnant, as shown through various blood test over the given time period. Help??