Uncertainty

Hi everyone, I'm 15 almost 16 and my problem is pretty awkward but I still want to know the cause. I've had dark circles around my nipples for a few months, but only when they're erect. I've been on the pill for a bit over a year now, and I'm not pregnant because I've been getting my period all this time. I'd rather not go to the doctor just yet, so if anyone knows a cause, I'd love to hear it.