Hello Doctor, I have had persistent dark spots and small acne in a specific area ( upper back) for several years, starting around the 2020 pandemic. Initially, I thought this might be due to sitting or lying back (without shirt) on a wooden chair for long periods, which I suspected caused stretch marks and acne. However, I stopped this habit a long time ago, but the spots and acne have continued. There is one particular blister that has been especially prominent—it tends to bulge and then flatten over time. This has been happening for years. I shower regularly, especially after sweating, and I haven’t experienced any other health issues related to this, so I didn’t pay much attention before. Recently, however, this prominent blister has become much larger than usual, which has made me concerned. I would appreciate your advice on what this could be and whether I should be worried. ( this blister is painful when touched , also I don't have any face acne or anywhere else)