It might possibly be contact dermatitis
which is an allergic reaction
to something your skin is in contact with. Also possible to have a fixed drug eruption
caused by taking something internally (ie: medication, birth control pill, food, or something ingested). Also possibly could be ringworm (a fungal infection). For relief, take antihistamines (Allegra, Benadryl, Zyrtec) daily and apply topical steroid (OTC cortisone too weak, stronger prescription strength steroids work better but need a doctor to prescribe) and also an antifungal cream. See a doctor or dermatologist
to perform a PATCH test to see if you're allergic to nickel or other metal or other chemical or ingredient. Another possibility is atopic dermatitis
or eczema which can be localized. This usually responds to topical steroids.