I don't know what you have, but could it possibly be an allergy to something you are eating? I'd say it's acne sooner than yeast.
Adults can get acne and the neck, chest, back area is usually the place they get it. Is your skin prone to oilyness? Are you taking any medication or have you tried any products such as body washes which are PH balanced? Adult acne is often as a result of hormonal changes/imbalances.
People can get yeast beneath their breasts or in folds of skin, wherever there is warmth and moisture. I've not heard of yeast infections coming out like spots (i could be wrong though).
Have you thought of asking for advice at a pharmacy or a health food store. They often can be more helpful with suggesting a product to try, but then again, if it's a hormonal issue, you will need to see a qualified doctor. Best of luck and let us know what it is if do find out.