Discolored Indentation on arm

[Dbrown47]

I recently noticed a discolored indented spot on my upper arm.. It is about 1 1/2 inch long and 1/2 inch wide. Very irregular shape similar to a birthmark. The color of it is a little bit darker than my skin color. I know this sounds weird, but if I press down on it, it feels like part of my musle underneath is gone. Just where the discolored spot is though. Its clearly indented; if I rub my fingers over it, there is definitely a dip in  my skin. I have never had anything like this and I'm only 24 years old, any experience or advice would be great. Thank you!