2 month old with white head on bottom

my baby had this pimple on his anus , we went to see his doctor and she told us to keep his bottom clean (OF COURSE) and soak him 3-4 times a day but it clears then it comes back , it came back twice since we visited his ped. we go in tomorrow for his 2 month wellness check up and shots has only one had this problem with their babies ??