H.B.
Don't put cornstarch on a yeasty diaper rash - it will feed the yeast and make it get worse.
I have had good results using any store brand Lotrimin type cream (clotrimazole) - smear a thin layer, covered by a thick layer of Aquafor at each diaper change. Then to maintain a healthy butt, a smear of Aquafor whenever a bit of redness appears, and at the bedtime diaper change.
Use just water on a soft washcloth to clean her at diaper changes, and pat very gently. Don't wait for the appt to start treating it, diaper rashes can get really bad (raw and bleeding) very fast if left without treatment. But they can also clear up quickly when treated properly. Good luck!