Hi dear , As per your query you have red mark under jaw of your baby . Well the probable reason for red mark under the jaw is body s reaction to heat . This occur due inflammatory response of body due to trauma . Usually such red marks go away by their own without any treatment . Apply cold compresses to the affected area and moisturizer . Prevent affected area from direct sun exposure . If condition doesn t get better or get worse the consult doctor or pediatrician for proper examination and rule out cause like allergic reaction . Doctor may prescribe steroid ointment or antihistamine medicine . Hope your concern has been resolved. get well soon. Best Wishes, Dr. Harry Maheshwari