This redness of your dog's anus could be down to a number of different health problems.
The most common problem associated with redness in this area would be an infection in your pet's anal glands - a potentially serious condition.
What does it mean when a dog's butt is red? Although it is most likely an infection, this will need a closer examination by a vet - who will be able to thoroughly investigate the problem area, and hopefully give you a fast diagnosis of the problem.
Once this has been done, the vet should then be able to commence treatment for the problem straight away.
Usually infections come in three forms:
Fungal - such as ringworm.
Bacterial - caused by bacteria.
Parasitical - caused by things like worms or other creatures.
Infections in this area will usually be treated by a course of anti-biotics given by your vet.
I suggest you take her to the vet and get it checked out asap they will probably give you ointment for your pup, don't let him lick at it that will make it worse if he does lick it you should put a doggy head cone on him so she cannot reach it, just until you can go to the vet. I hope I could help you. Good luck