Keep an eye on it
What it can look like. A general, vet-reviewed example — signs vary and this is not a diagnosis. When in doubt, ask your vet. 
Chewing & biting at skin in dogs
Clinical name: Self-trauma
Gnawing or nibbling at the skin, sometimes leaving damp or raw patches.
Where it shows uppaws · tail-base · flanks
Typical patternTail base (fleas), paws and flanks.
Linked allergy typesFlea allergy · Food allergy · Environmental (atopy)
Food vs environmentalTail-base focus leans flea allergy.
What you can do
Chewing at the tail base strongly suggests fleas — check and treat for fleas first.
Don't confuse it with
Other causes a vet will want to rule out: fleas, pain, mange.
This is educational information, not a diagnosis or treatment plan. Always see your veterinarian about your dog's health.
Related signs
Itching & scratchingKeep an eye on it
Frequent, restless scratching that the dog can't settle from — the hallmark of allergy.
Excessive lickingKeep an eye on it
Repetitive licking, especially of the paws and legs, often to the point of wet fur.
Face rubbingKeep an eye on it
Rubbing the face and muzzle on carpet, furniture or with the paws.