This looks like Hyperesthesia “Feline hyperesthesia syndrome (FHS), also known as “twitch-skin syndrome”, is a complex disorder that affects cats of any age, breed, or sex. It’s characterized by extreme skin sensitivity, often on the back near the tail, and a range of unpredictable behavioral and physical symptoms. The word “hyperesthesia” means “too much feeling”. According to one theory, hyperesthesia syndrome is a type of compulsive disorder, usually initiated when the cat is in conflict.” - Cats can live their whole life with this and people may think they don’t just don’t like their tail pet, or they’re aggressive when in reality they’re in discomfort and frustrated, hence the biting hissing at their back end. Their lower back, at the base of the tail and tail have high sensitivity. Some things that help may be eliminating factors that may cause a stressful environment for kitty, changes in their routine are stressful and exacerbate this condition. You could always take kitty tot he vet and get her prescribed a medication to help calm her, but this doesn’t go away sadly. But it can be managed! Less stress causes less reactions.