It could be a bug bite, it it gets red, swollen, hot and painful please give your doctor a call.
You could put calamine lotion on it. Thats safe to use while preggers. If that doesnt clear it up and your worried it never hurts to get it checked out by your doc.
I had one with my last pregnancy but this one looks alot different.
I had the same thing and it hurt its kinda like a pimple the doc told me its normal with all the extra hormones and stuff goin on now and that your breasts get really hot/sweaty so its the gross stuff gettin out. Hope that helped. Could just be a bug bite too lol.
I am 32 weeks. It just appeared over night