What Causes Brown Skin Discoloration On The Face?

My 16-year-old daughter has a new, small brown spot on her face (cheek). It is maybe 1/4 inch in diameter and appeared suddenly in the afternoon. It was not caused by any injury. It is not raised like a mole, it doesn't look like a bruise. It is just a brown discoloration of the skin. What could cause something like this? It is very obvious, and she is very upset about it.