What Causes Ring Around Mouth Darker Than Skin Color?

I have just developed a ring around my mouth that is slightly darker than my skin. I am a 29 year female. Other info that may or may not be helpful is that I have donated blood within the last month and am sick with a fever that has reached over 103 within the last 24 hours. And the ring showed up shorty after an episode of high fever and a sensation of hot or burning lips.