The case of the missing period..

So my soon to be 17 year old daughter hasn't had a period in six months. I took her to the doctor and they dismissed it and said it was natural. Because she is not sexually active. Well, it's freaking her out and it is causing her to gain weight and just over all its concerning to her. I'm not sure how to handle this because the doctor won't give her birth control to regulate period. Suggestions please or if you have a similar experience please share how it was resolved.