One possibility is Lyme disease, especially if the rash slowly expands after possible exposure to ticks and is not very itchy. Another common cause is ringworm, a fungal infection that typically creates a red, scaly, itchy ring with a clearer center.
Other causes can include pityriasis rosea, insect bites, allergic reactions, or contact dermatitis from soaps, plants, clothing, or other irritants. Taking photos of the rash over time can help a doctor see whether it is spreading or changing.
Seek urgent medical help if the rash appears with breathing difficulty, facial or throat swelling, very high fever, severe pain, confusion, purple bruising, or a stiff neck. Otherwise, contact a pediatrician if it continues spreading, lasts several days, or is accompanied by fever, fatigue, swelling, or recent tick exposure.
