What causes red spots under both breasts?

Yeast vs Bacteria: Often the red spots/bumps/rash under the breasts is due to excessive moisture accumulation and a resulting infection with either yeast (candida albicans) or bacteria (like staph or strep). Your doctor can easily help you with this. For now, try some ketoconazole 1% otc cream for the yeast and a benzoyl peroxide wash for the bacteria. Get in to see your doctor, they can diagnose the problem.