I assume you aren't breast-feeding or you would've said so. Are you a runner or do work or other kinds of activities out in the heat? Nipple chafing and fungal infection is common when the bra is damp from perspiration and/or is ill-fitting. Make sure your bra fits well and is made of a moisture-wicking fabric. Wear a clean one every day. If you have only one, make sure to wash it nightly in very hot water so it's clean when you put it on again the next day.
If keeping the area dry doesn't help heal the problem, you can try applications of a mycostatin, clotrimazole, or myconazole cream daily for at least 2 weeks straight. These are available over the counter in remedies made for athlete's foot, jock itch, vaginal yeast infections, and ringworm. All of them are the same thing; just look for mycostatin, clotrimazole, or myconazole as the active ingredient. If that doesn't help, you should seek a doctor's advice.