My Period Is On And Off, Could It Be Caused By Ovarian Cancer?

So I am 15 and started my period almost three years ago and up til 2011 it has always been the last week of every month and I never missed my period.

Starting January I didn't get my period at all, the same goes for February and March. After that I
would get my period off and on, sometimes the first week sometimes the middle of the month but for the most part I always had my period for five days. The last two moths though I get it for only a couple hours before it stopped and this well happen two or three times a month.

I recently heard that this could be caused by Ovarian Cancer as well as high stress or a change in diet. Now since it has been nearly a year now I know it couldn't be from stress and I did become a Vegan two months ago but by now that shouldn't be a problem. I do plan going to my doctor as soon as possible but I was wondering what the possibility's are and how worried I should be?

I would also like to add I have not had sex so I could not be pregnant.