Why is my dog chewing at her butt?

If you get a fine toothed comb and comb the hair on to a piece of damp wet paper, you may notice black specks that bleed red. Thats flea dirt. Get some Advantix 2 from your vet, dont use Frontline or OTC flea preventatives. They just dont work.

If its not fleas it could be worms or anal gland problems and they need a vet. Take a fresh stool sample with you for the vet to check as there are different kinds of worms.

It could be fleas , worms. I would take your dog to the vet they can tell you why your dog is chewing at the butt.

Fleas

could be impacted anal glands, the groomer or vet can help with this. If you take her to a groomer to get the anal glands expressed, you will be required to bring proof of shot records.

Read some articles such as these (I haven't read this particular one myself... it was just the first Google link that came up, but I'm sure it will explain a thing or two)

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http://pets.webmd.com/dogs/anal-sac-disease-dogs

Fleas, worms, impacted anal glands, skin infection, hot spot, allergy of some sort, etc.......no one here can tell you exactly why.

Give her a thorough bath.

Take her to the vet and find out.