What Causes Discoloration Of Skin Around Nipples?

I am 17 years old, i weigh 8 and a half stone and have no medical history. My problem is that the edges of the circles round my nipples have faded in colour. They are no longer round and the faded colour is much closer to the colour of my skin rather than my nipples. I find this very unusual, therefore it has majorly lowered my self-esteem. I would really appreciate a response from this as it has strongly affected my life. Thank you.