I Have A Black Mark On My Chin. I Am

I have a black mark on my chin. I am 15 years old and I haven't had any sexual relations with anyone. I started seeing this spot around 2 months ago. I shower everyday or sometime every other day. I tried really scrubbing hard on the spot in the shower and it started to fade and eventually went away. I have used creams and vitamin E oil to hydrate my skin and I keep my face clean. I don't drink pop that much. Only on occasions. Everything I eat most of the time is home made. I know how to treat this issue, I just don't know what causes it. Please help