Itching in the rectum only 13 please help?

I'm 13 years old and I am a girl. I live with my grandmother. I have told my grandma about this itching but she seems to think it is because I don't wash enough. I take a shower every night though. It feels as if something is wriggling or crawling every time I sit or lay down. When I stand up, it's just an irritating itch. Maybe too much information but I really want to know what is causing this. I can't go to the doctor's by myself and I don't know what to do. This is really getting on my last nerve and I want to make this irritation stop. Any help please?