Is she due for some more teeth? Maybe this is the reason?
I would just suggest lots of nappy off time - specially with the summer coming, this should be easier.
I dont use zinc cream - I used a calendula based cream, and there is a rash cream and a balm for protection. Just make sure that you put some sort of barrier cream on at each nappy change and hopefulyl if she does poo, it wont stick toher bum so bad.
A dom midwife also told me that they can build up a resistance to zinc creams - not sure how true that is.

Fi