Why Am I Not Having My Menses?

Im 13 years old and haven't had my first period yet, recently I found blood coming from my anus, I'm 99% sure it's coming from my anus, every time I do a poo it is there, I'm really scared, I don't know what it is, it is red blood and when I wipe, it is on the tissue. Please help me, I haven't been to a doctor yet,