What's wrong with this pimple growing on the buttocks? look

Hello, according to your photo, your disease is folliculitis;

Folliculitis is a purulent inflammation caused by bacterial infection of the whole hair follicle;

The main symptom is red papules, which then turn into pustules. Adults mainly occur in hairy parts, while children are prone to head injuries. Their rashes sometimes merge with each other, leaving small pieces of hair loss after healing.

Antibiotics can be used as appropriate, and 1% neomycin ointment, mupirocin ointment, fusidic acid ointment or 2% iodine tincture can be used externally.

Because it is difficult to master the dosage at home; I suggest you go to a specialized dermatological clinic and make a better treatment plan after careful examination;

I wish you a speedy recovery and get rid of the troubles caused by the disease!