I would take her to the pedi to rule out any infections.
That said, it could be the wiping issue, yeast infection, etc.
My daughter and I cannot take a bath or we will get an infection, just like clockwork. We've been a shower family for years.
Another thing that helps is to use the blow dryer on low after shower/bath to completely dry the area, we sleep with no undies to air out and all that helps too.
Any change in soaps can mess us up as well.
If it is yeast, she made need an antifungal med. We both drink a DanActive daily and since we started that routine, the occurnaces have been super low. I buy it at Costco in packs of 24 for about $12 each. It is a great product for us, no more infections etc.