Thirteen days ago on November 22, 2019, I had these strange circles on the thumb side of my right forefinger. They are exactly where the finger presses against a coffee cup finger-loop, so lately I have been drinking my coffee with the middle finger doing the lifting.
It is photographed in strong side light to show the details better. It doesn’t hurt anymore and never did get red from infection.
I worried a bit that it might be an insect laying its eggs in my flesh, but nothing came out.
Probaway - Life Hacks 
Perhaps a burn blister from a hot cup of coffee? It is easy to ignore small injuries in old age because there are so many and so often. They take twice as long time to heal as they did when you were just a sprout.