What To Do For The Dark Circle In Chest?

Hello, about 2 months ago i noticed a dark circle on my chest, it was a little darker than my skin color so it stood out, it didnt itch or sting or anything, and is not raised from the skin, so i didnt think of it as a big deal, but now two months later ive developed many of these spots on my chest, the sizes vary but they are spreading pretty fast. I dont go out shirtless in the sun often at all, what could these be?