could she be having a reaction to her diapers, or wipes? i've been using the same brand of wipes with my daughter ever since she was born (she is turning two this month), and all of a sudden she is sensitive to them and got a yeast infection because of it - not sure the technical reasons why, but when i quit using those wipes, she started getting better, but when i started using them again, she got worse again!
also, i think using baby powder had something to do with making it worse too!
she cleared up fairly quickly when we switched from nystatin to lotrimin (antifungal cream - ask your pharmacist about it), and started using totally rinsed out baby wipes (or a wet paper towel, or a baby washcloth), and we use the hair dryer on her bottom before we put her diaper back on.
BUT, i'm with you. i think this is my daughter's 3rd yeast infection in less than 6 months and that's too many in my opinion! if you come up with any other ideas of why, let me know!