Am I starting my period?

I am 13 years old and I have not had my period yet. I noticedbrown stuff in my underwear yesterday and I changed my underwear then later found brown stuff in that pair too. I stuck a pad in overnight but found nothing this morning. Then I went pee and wiped and found brown stuf on the toilet paper. Then I later found brown stuff in the pad. Is this my period? I have not really had any symptoms except I have had a slight stomachache. If this is my perid what should I do? I am kind of scared.