From another large breasted woman...
I have to be extra careful in the summer with the extra sweating to change bras daily and to use lots of powder, otherwise I will break out on my breasts like you have described.
Also, wear a clean PJ top every night, as my boobs sweat at night too.
Use only mild soap and and light lotions.
Always shower after swimming.
I find that my breast trap everything from sand to crumbs and those items can become an irritant.
I would say that I always have small red dots / pimples on my breasts. I've had dermatologist look at it and they have never said it needed to be treated.
I will often apply Psorian or other OTC skin creams to the area and it often clears up, but quite common returns during the warm months.
I would recommend you google Petechiae and ensure you don't have anything similar to that condition, as that is a condition that requires immediate medical assistance.
http://www.mayoclinic.org/symptoms/petechiae/basics/defin...
If you're not extremely bothered by, wait until you start your cycle and see if it clears up like other hormonal break outs.
Also, take a picture once a day if it continues to worsen so you can track it with real pictures, versus your memory of what it looked like when it started. It's easy to dismiss how it progressing or not when you see it up close.