It’s not entirely clear what causes this rash. Something may be irritating your skin, such as a skin care product or toothpaste. It’s also possible that you’re allergic to something that’s touching your skin. Many people develop this rash when they apply a corticosteroid medicine to their skin for too long. That’s why it’s so important to follow the directions for taking medications. Be sure to follow the directions on medications that you can buy without a prescription and your doctor’s instructions for using all medications.