What are these red marks on my chest?

When I first spotted them I thought they were pimples but they're not bumpy or anything, they're just red marks, they go away and then they come again. They come around only one of my breasts and sometimes in the middle of my chest and are there for a day then disappear and come back again. BTW I'm a fifteen year old girl. I'm not too worried about these markings I'm just not sure what they are.