Thanks for the additional information.
Does her skin lesion look like this?
If so this may be what is called an eosinophillic granuloma.
Underlying causes include flea bite hypersensitivity, inhaled allergies or atopic dermatitis, food allergies, feline demodex, Cheyletiella mites, Ear mites (Otodectes ear mites) that have spread out from the ear, ringworm and less commonly bacterial infections. Rarely these lesions can be secondary to a type of skin cancer called cutaneous lymphoma.
Because she doesn't go outside contagious causes like demodex, cheyletiella, and ear mites are less likely. She is a bit young for cancer although it would be a rare possibility.
Fleas bite hypersensitvity is possible as it only takes one bite a month for a sensitive cat to have lesions and fleas can get to even indoor cats with open doors and windows in the summer. Inhaled allergies, food allergies and ringworm are all also possibilities.
Things that you can try at home to control allergic symptoms include antihistamines and omega 3 fatty acids:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at 1mg per pound every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds.
2) Chlorpheniramine at 4mg once or twice daily.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve her
allergic symptoms. I like 3V caps or Derm Caps. Together antihistamines and omega-3 fatty acids work synergistically. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
Food allergy is another reason for the lesions that you are seeing. You might wish to try a hypoallergenic food for 8 weeks or so. My suggestions are Hills Z/D or Royal
Canin Duck and Green peas. She will need to eat only that food, no other treats
or foods during her food trial.
If she isn't improving then she should see her veterinarian for a fungal culture, skin scrapings or a biopsy to definitively diagnose her condition.
Please let me know if you have any further questions.