Perhaps it might be due to urine or poop or the nappy material. You need to consult doctor and get a better cream. Meantime, stop using whatever you are currently using to avoid further rashes and blister. It might also due to too much of product applied on the bum area causing it to not heal properly.
i think better to use warm water to wash if she poop and dry properly dont use wipes or anything first until it gets better.