The steroid will either clear you right up in a few days (confirming the nummular dermatitis diagnosis), or make the rash get much worse, also right away (if it's fungus.) Nummular (meaning round) dermatitis is much more common than fungus ("ringworm", so the dermatologist is probably right. But you don't need to worry about masking symptoms. Your clinical improvement should clinch the matter.
Take care.
Dr. Rockoff