@_hm: yes. Do not delay. One route is to go to your family doctor who may then refer you to a specialist -- or (at least where I live) you can call a dermatologist directly. One thing is that dermatologists are often booked up and you may have to wait a long time to get an appointment -- if your doctor refers you to a specialist and your doctor thinks it's important enough, then he/she can get you a priority booking and speed things up.
The main thing is to get the ball rolling as soon as you can. It may well be nothing important, but it may also be something that is best treated right away. Please let me know how you get on (Max.firstname.lastname@example.org)
@Max: Thanks. I was just planning visit to my family physician. I too suddenly observed enlarged drak brown skin under my arm. I was wondering what is that. I should soon first approach my family doctor for next action.
@Max -- "Nobody should lay their hands on my Nuts" -- Testicular cancer is another item that is important to screen for -- but the screening can be a bit painful if done by a female medical professional with long fingernails