Thread: Only one period this year... what could be the problem?

I've had periods since I was about 12 or 13, and they have come pretty regular. Well this year, I've only had one and that was back in August. I have missed periods before, but never more than two months, which that was only one occasion until this year. I have done a lot of studying up on what could be the cause, but I'm still lost on what is causing my absense.
Here are a few things that I think it may be, and what I've done to try to help:

-stress (I've taken time to relax, and tried not to worry so much. This year has been pretty stressful for me with school, work, and other things.)
- Weight Loss ( Last year I lost about 90lbs. I have tried to incorporate more fats, calcium, protein, and other important dietary needs. I know there is possibly more I may be missing. I have reached my goal weight and have been approx. the same weight for the past seven months.)
- Hormone Imbalance ( I'm a teenager, so it could happen)
- Low iron ( I've been getting cold easily, but I have been taking prenatal vitamins for the iron content, which seems to help.)

Please, if anyone could help, it would be great. I know seeing a doctor would be a good idea, but right now I really can't afford it, unless it's absolutely necessary. A few other things to know is that I've never been sexually active (so I'm not pregnant) and I have an aunt how had endometrial cancer.

I forgot to mention that I'm 17...