Liz:
Most likely you have mild nummular eczema. "Nummular" means, round, coin-shaped. This is not an allergy; rather, it comes and goes for no special reason. It can go away on its own, as you've seen yourself, but hydrocortisone helps it along. Tey the nonprescription form (1%). If that doesn't help, a doctor can give you a stronger variety.
Best.
Dr. Rockoff