No one can diagnose a rash over the internet.  In fact, even doctors who are trained and actually looking at a rash, may not be sure what is causing it right away.  Since the red places are turning up on both breasts, it doesn't sound like inflammatory breast cancer.  These could be some type of insect bite, allergic reaction, or irritation.  Heat rash is also common at this time of year, especially on the warm, moist underside of the breast.  I don't think you need to worry about this yet because it's only been three days.  Be sure to mention it to the doctor when you go for your appointment.  If the spots get worse or if you develop new symptoms, you might want to see if you can have your appointment sooner.