What Causes Pus Filled Blisters In The Anal Region?

my daughter is 22 years old who use sanitary pad for whole one day and after that next day she has itching on anus and after 2 to 3 day she has small pimples 2 or 3. and one of that like size peas. and when touched it paining and pus in it. some pus came out. please help us doctor.