K.W.
Both daughters have had this. My toddler also had it recently, likely due to antibiotics for an ear infection. My doc said to use clotrimazole (sp?), which is in both yeast infection creams and in athlete's foot creams. The cream for athlete's foot is the exact same and is way cheaper. Ask the pharmacist, they'll help you pick something out. Just put the cream on when you change her (or after she goes potty, if diaper trained), and it will eventually go away.
Good luck!