Do the bumps look like little, um, weeping mountain caps? It could be a yeast infection. That can be caused by high acidity from juice, maybe too much of a particularly acidic fruit, or if she's had diarrhea. If she was recently on antibiotics. She could have a corn allergy.
Do NOT use cornstarch powder or feed her corn products at all. If it's yeast it'll feed a yeast infection. They like sugar too, so no sugary foods or drinks.
You can get Phillip's Milk of Magnesia, plain, and mix it with Balmex or Desitin into a paste and apply it right to her bottom and coochie area to help soothe and reduce the acidity. If it turns out to be yeast you can apply .1% (or is it .01%?) Lotriman cream and mix it with the pasty mess. Eating yogurt or applying plain yogurt to her affected area can help soothe too.
After a bath let her air dry. Try to let her have diaper free time and naked time nothing on for a while after she's relieved herself and before applying the pasty mess and a clean diaper. Change her diaper frequently and don't let her remain in poop at all... no wet diapers if possible.