Have you started using any new hair products - conditioner, serum, leave-in stuff?

I used to have small acne on my chest, doctor only advise the normal stuff like wash your skin and put benzoyl peroxide etc. After few years I figured out that it is the hair products that cause the acne.

Even after washing off conditioner, the water has oil residue and can cause acne when drip on your hair and butt. Try to wash hair away from your body, use separate towel for hair, avoid too many hair products, change your pillowcase etc.