What Causes Brown Spots Under The Breasts?

I have brown spots under my breasts that look like birthmarks. They seem to spread over time, it started out really small and grew, over months. I have 2 kids, one two years old. I have thought that maybe it's from pregnancy. I am not at risk for diabetes, I am a healthy BMI. However I do have hypothyroidism, so I accept that it could be caused from hormones. Just would like to ask. Thanks for your time.