Back acne

Hi 35F, this year I’ve been experiencing pimples (I think) on my upper shoulders where I have never got them before. They are itchy. They come and go quickly and don’t feel like pimples. The first time that I got them, I blamed my friends hot tub. But it has cleared up and reoccurred since then. I shower after workouts and regularly and generally am not sweating much. I take long baths, I thought it might be some sort of build up from that since it’s about the level of where the water is. I thought maybe it’s yeast related but I don’t understand how.: so current treatment is head and shoulders left on for a few minutes. Wash off, dry off, antibiotic ointment, and moisturize: I’ve been asking my friends and I’m shocked with how many of them have similar experiences ! They’re all treating it in different ways though or blame different things .:: hormonal is the most common but one even had shingles at 33 I love my back and have tattoos so I’m getting really concerned about the damage, I have a doctors appt but is this common at 35? When I’ve never at it prior?