I have a red rash just under my breasts. not sore or itchy but embarrassing. what is it?

Several things: This could be a heat rash or a yeast infection. You should have a professional see it.

Possible yeast: If it's solid, pink, moist and itchy it is probably yeast, a common infection in large-breasted women. You can try some otv anti fungal creams, those recommended for vaginal yeast infections or athletes foot. If not better within a week see your doc.