What Does Hyper-pigmentation On The Left Rib Cage Indicate?

Hi, I ve had what i ve thought to be just normal pigmentation on my left ribcage for about a year and a half - two years and haven t really ever thought anything of it, but recently I ve noticed it to feel like a bruise and it definitely looks like one too. I don t have pigmentation or anything anywhere else on my body so I m not sure if that s normal. I m 16, healthy as far as I know and am physically fit but have been under investigation for potential problems with my heart (have had echo, holter etc) all of which came back fine except for an increased HR at points during the day (up to around 200) and an episode of atrial tachycardia. I am not on any medications but have been in the past (betablockers and calcium channel blockers), which did slow my heart rate but I find I can manage without them, however I do often get very heavy palpitations which are visible in the arteries on my neck and can see depressions and vice versa in sync with my heart beat. Just wondering if it s anything to get checked or of significant concern as have only thought of it to be normal discolouration of my skin