it's normal to have really irregular periods when you first start. My situation was just like yours. Enjoy it while it lasts, after that I started getting it twice a month. I eventually got on the pill to regulate it.
ThatGirl, are you thin? I was really thin at your age, about 90 pounds and 5'2", and I had spotty periods.
I would think seeing a doctor would be a good thing, but this may be normal for you until you get a little older. There's no real cause, at your age, to regulate your period. Normal comes in different forms.
Best wishes.