Yeah, definitely the back of arms is probably KP. I see my derm on a regular basis, and she says it's common (and all my friends have it). She prescribed some lotion for me, but I think the idea is to keep it clean and moisturized. Use a neutral soap that doesn't strip your natural oils too much - like Cetaphil.
As far as back/chest/etc. acne or bumps, again use a mild soap, nothing too harsh or fragrant. I love Cetaphil... Also, try to wash towels after use, and a fresh washcloth always. Clean sheets, loose clothing, etc.
Maybe it's all those Cathe vids causing sweat! JUST KIDDING -- I'm definitely one to know about skin issues though.... Take care of yourself!
(Not trying to give med advice...just sharing the wealth from my own experience and what my derm says)
Take care,
Caroline