RASH ON FACE. A rash on one side of the face could be a number of conditions. What comes first to mind is shingles. Does it itch? Does it hurt? What should you do? See a dermatologist or your general doctor to examine you and then let him/her make a diagnosis and treat appropriately. Don't wait.
See a doctor. If this concerns you, you should see a doctor. It is impossible to diagnose somebody over the internet.