Has the doctor checked out her rash? It's possible that what she has is actually a yeast infection. When my now 4 year old was a baby she would get diaper rashes so bad they would bleed. And they would come and go. Turned out she was somehow getting yeast infections. Usually something such as Nystatin (Rx only) worked. And when I ran out of that, I used the monistat cream on her.
I also had to keep changing diaper brands on her because of it. Seems she did better in Luvs then any other brand. And Pampers was the worst for her. I even tried stored brand, but those made the rash go around her legs.
I now have 7.5 month old twin boys. My first boys too. And they started getting a diaper rash. I have this stuff that is called APNO--All Purpose Nipple Ointment. It's another Rx thing, but you can make your own. It's an equal mix of Monistat, Neosporin, and Hydrocortizone. I was Rx'ed it for nipple thrush and it's in such a huge jar. When one of my twins broke out over night in a terrible blistered rash I used this and by the end of the day it was cleared up.
And it really is All Purpose. I've used it on scrapes, cuts, burns etc. I even used it on my 8 year old daughter's finger when she managed to get it infected around the nail. Put it on once (and by this point it was looking bad, swollen, red, white and had pus) in the morning, applied a band-aid so she didn't wipe it off while playing, next day her finger was cleared up! Still took her to her Dr. Appt. and he was amazed that it was totally cleared up.
Another one that works is to mix equal parts cornstarch and Maalox to create a paste. Stay away from baby powders. Try to limit sugar intake (from juices, snacks, etc), and avoid acidic drinks, as they could cause her urine to "burn" the rash (kinda like sting..think of a paper cut when you get soap in it).
Good luck! If I remember some other things I have tried i"ll pop back over and let you know!