Have you lost weight recently? Have you become a lot more fit recently?
I have never had problems with my period except once. For ten years I got allergy shots until they stopped working. Went to the allergist and he gave me a shot of cortisone (I think that's what it was) in the butt to make my allergies better.
Well, my allergies got better, but then I had a period for six weeks. Just constant dripping. Finally it went away. But I'd worried for about a month before I made the connection. When I said something to the allergist he was like "oh, yeah, that is a side effect."
The point is that you may have had some change in your medical or physical or emotional life that you haven't connected that is the cause of this.
I believethe real worrying is better saved for those who have lots of pain and lots of period, not a lack of period.
Good luck at the doctor.