I'm 14 and on Friday I had abdominal pain. The on the weekend I got diarrhea and so I tghought that the pain was just a stomach ache. Then on Tuesday I went to the toilet and the vaginal discharge was dark brown and not clear like I'm used to. I asked my sister but she said that it wasn't period but the next day, I still has it. Is it my period?

It is part and parcel of your period. Brown discharge is old blood. I would talk to your mother about this and not your sister. Your mother has had many years of experience dealing with all sorts of stuff that comes out of her Vagina. She will help you.