This behavior is called "mowing" informally, but any type of hair loss is referred to as "alopecia". It can be caused by
- Parasites (fleas, mites, ringworm)
- Allergies (food, seasonal, environmental)
- Dry Skin (nutrition deficiency or environmental)
- Psychological (boredom, anxiety)
It's likely (based on what I've read and my personal experiences with mowing) that the shot was cortisone.
For this trial, the patient receives some kind of cortisone-type treatment for 3 to 4 weeks. At the end of that time one can tell if the mowing has improved (and the mowing is said to be “steroid responsive”) or the mowing has not improved at all (and the mowing is said to be “steroid non-responsive.”) Knowing whether or not the mowing is steroid responsive helps classify possible causes. For example, seasonal steroid responsive mowing is most likely to be from fleas (technically allergy to the flea bite) or an airborne allergy. Non-seasonal steroid responsive mowing is most likely going to be from a food allergy. Steroid non-responsive mowing is most likely to be from a parasitic or fungal problem or a food-allergy. Many experts like to do the steroid trial at the beginning of the work up while others wait until more results are in.
(Source: Veterinary Information Network)
Generally, when I've worked with veterinarians on my cats' skin issues, we'll rule out all of the possible physical causes (do a flea treatment, food trial/allergy testing, etc) before deciding that it's psychological. If you felt that your vet skipped a step or would just like to see someone who specializes in skin diseases, you can ask for a referral to a veterinary dermatologist. They are slightly more expensive than a normal vet (the one I saw charged double my vet's normal rate for the initial exam, but follow up exams were about the same amount as my normal vet's exams).
If you do decide that it is psychological, the VIN article suggests:
This is generally called “psychogenic” mowing. We don’t imply that we know if cats are licking out of obsession or out of anxiety or even boredom. We simply say that there is nothing wrong with the skin. Psychoanalysis is generally unnecessary; the approach is aimed at environmental enrichment. This means the cat gets more toys, more games (feeding in a different location daily to create a hide-and-seek sort of cat entertainment), and more attention. Clomipramine has both anti-anxiety as well as anti-compulsive effects and has been helpful though it does not come in a convenient feline size and may have to be compounded. Amitriptyline has both anti-anxiety properties as well as anti-histamine properties and is sometimes used to cover both the medical and psychogenic causes of mowing simultaneously.
We have put one of our girls on Amitriptyline (she definitely has anxiety, and it was affecting her bladder as well as causing mowing). I definitely recommend getting it compounded into a transdermal gel if you decide to go that route, as an anxious cat doesn't need additional stress of being pilled.