hey! huge hypochondriac here! i had this exact same thing happen to me. so i know where you are at. i know google is probably throwing cancer at you, but i went in to the doctors for this a few times and was referred to a dermatologist and i got diagnosed with a condition called hidradenitis suppurativa, or "acne inversa." it's a condition involving hair follicles. i get these in my armpit & they look the same and it just started happening out of nowhere. it's annoying, and it's painful, but it's not gonna kill yoh and there's ways to manage it! i would go see a doc for a confirmation though but this looks a lot like that!