T.-
As much as I'd love to offer help, I've learned from our pediatrician that all skin conditions are not easily diagnosed and treated. I'd highly recommend taking her into the office to make sure you're treating it appropriately.
If it is a yeast infection, it's fungal, and will need to be treated differently from eczema or dermatitis.
The advice to stay away from powder is consistent with the guidelines from the American Academy of Pediatrics because of their respiratory systems.
I'd at least call the pediatrician's office - they may be able to make a recommendation over the phone or may as you to come in for confirmation of cause before recommending a treatment.