Why is dog biting back tail. Pet's info: Dog | Mixed Breed | Female | unspayed | 1 year and 9 months old | 30 lbs

This would be a typical spot for fleas to bite and congregate, so if you have not done any flea protection you will need to start with that now. There is a possibility that your dog is suffering from a flea bite allergy, and getting rid of the fleas will alleviate that. The coughing fits/vomiting will need to be investigated by a veterinarian. This could be caused by kennel cough or another upper respiratory infection or a heart condition. I suggest you schedule an appointment with your veterinarian for an exam.

The most common reason for biting near the back legs is fleas or flea allergy dermatitis (which is an allergic reaction to flea bites). I would recommend that you have Sophie examined by the vet to see if she has flea allergy dermatitis, since she may need antibiotics or anti-inflammatories to help clear the skin while you work on preventing further fleas. She needs to be on a good quality monthly flea preventative, your vet will know which work best in your area. If it is a food allergy, just changing from one food to another may not be good enough. Some dogs need to be on a prescription hypoallergenic dog food for a couple months and then slowly add in other foods to see what she could be allergic too. Or perform allergy testing. Dogs don't often get ear mites, she may have an ear infection caused by the allergies or just feels itchy on her head.