Different reasons could cause different skin rashes, but they could also be cancerous.  Sometimes a rash is just a rash, but it can develop and grow into a cancerous one where it grows larger and becomes shinier over time. An indentation might form in the center, where it may begin to ooze or bleed. If you see a rash developing, don't delay visiting a dermatologist because even if it isn't a cancerous one, it can become severe and cause more trouble later.