Incase it's happening on daily basis please see your doctor for appropriate remedy do not try any home remedy . For temporary relief keep that area moisturized.

Lipeeka it would be due to dehydration , which causes reduction in the skin moisture and in the anal are is more prone to infection. there are two ways to look at the problem with a solution to this is, 1. Increase her water intake along with the fibre which will help the skin to have a good moisture content. 2. Please see a dermatologist who can help you with a anti skin allergy creme which you need to apply externally which will reduce the infection.