Dark brown and very very dark green stool?

I keep having dark stool (Dark brown soft feces and today a very dark green and after i go to the restroom my butt hurts a lil). Other than that I haven't had any other problems, it has been going on for about a week and I was just wondering why it is so dark. I don't it's iron causing it because I don't take iron supplements. Does anyone know what this could be?