I havnen't had my period in six months

I am 18 years old. I got my period when I was 12 and I have always gotten it every other month. My mother used to tell me that it was just irregular and would regulate itself out. I have missed one cycle before but I have never had a time period this long without menstruating. I am concerned because it will be six months in March. I do feel pressure and extreme fatigue and I am not overly stressed. I was just wondering if anyone had any idea's about what it might be. I went to a gyn. but since i'm not sexually active he did not do/check anything except give me birth control. He told me to start when I got my period but I still haven't gotten it. I'm just nervous that there might be another problem and that is why i'm not getting it. I am also looking for a non-hormonal way to help my body regulate. If anyone has any idea's or reasons that would be great!

[AnnieBrooke]

Are you underweight?  Lack of body fat can cause periods to cease.  I've also heard that stress can do it, but haven't ever been able to prove that one personally.  lol