I'm sorry to hear of this with Josie. You appear to be describing a skin condition called military dermatitis - a papulocrustous (crusty bumps) eruption seen mainly secondary to allergic skin disease. In rarer cases we'll find the demodectic mange mite which can be found upon examining skin scrapings microscopically.
Whenever the area between the edge of our cat's rib cage to the tail is involved, a flea saliva allergy is most commonly found (these bumps can be found around the neck and elsewhere on the body as well). Even if you can't find any fleas, it's best that you apply an effective over the counter flea spot-on such as Advantage/Advocate or a fipronil-containing product such as Frontline or, preferably, one of the newer prescription flea spot-ons available from Josie's vet. In severe cases we can either inject or orally administer a prescription antihistaminic corticosteroid such as prednisolone which often works wonders in these cats. Antihistamines such as diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton) aren't reliably effective against allergic skin disease but you might see how a dose of 2mg of an over the counter chlorpheniramine once or twice daily affects Josie. Please note that cats can be such effective groomers as to remove all evidence of fleas. It's important to still apply the flea spot-ons.
Less commonly but still important, Josie's miliary dermatitis skin can result from a food intolerance or atopy (allergies to environmental allergens such as pollens, molds, dust and dust mites, e.g.) Food intolerance/allergy is addressed with prescription hypoallergenic diets. These special foods contain just one novel (rabbit, duck, e.g.) animal protein or proteins that have been chemically altered (hydrolyzed) to the point that her immune system doesn't "see" anything to be allergic to. The over the counter hypoallergenic foods too often contain proteins not listed on the label - soy is a common one - and these proteins would confound our evaluation of the efficacy of the hypoallergenic diet. The prescription foods are available from her vet. There are many novel protein foods and a prototypical hydrolyzed protein food is Hill’s Prescription Diet z/d ultra (my preference). A positive response is usually seen within a few weeks if we’ve eliminated the offending food allergen. I would find an emerging food intolerance unlikely in a 16 year old.
Atopy is usually initially addressed by the same prednisolone I mentioned above and used to address a flea saliva allergy. Prednisolone can be used as a test when differentiating a flea saliva allergy or atopy from a food intolerance. Both a flea saliva allergy and atopy are usually quite responsive to prednisolone. Not so a food intolerance. Please respond with further questions or concerns if you wish.