Get your dog to the Vet right away. Your dog could have something that is really bugging her and she's just trying to get rid of it. When dogs have things like mites on their bodies they tend to scratch, itch or bite in order to get the bugs off.
She may have a skin allergy. Check her food, if corn is one of the first three ingredients, consider switching to a dog food that has a meat (chicken, beef, etc. ) as the first ingredient. If it persists, see a vet.
When a dog chews its tail, most likely it has flees or an insect bite. By chewing the tail, the spot gets further irritated causing the dog to chew more.
