Are there other symptoms present such as itching or pain? It would help if there is a picture present. There are a lot of differentials such as an insect bite, allergic reaction, infection, irritation, or other skin conditions such as dermatitis or eczema. Treatment usually depends on the underlying cause. Have this checked by your doctor for proper evaluation. Direct clinical examination is important in determining the diagnosis.
Take care and keep us posted.
could be ring worm.... starts off as a "ring" shaped red mark with a bumpy and raised outer ring and then spreads outward until it eventually dries out and fades