hi Matt, don´t know if you have solved your problem yet. i had the same problem as yours and after clinical test, i have found to have the Stapholicoccus aureous.
it is important to control this area even if not very "nice" to show it
This could be an irritation, dermatitis or inflamed hair follicle. Are there other signs or symptoms present like itching,pain o r presence of pus? Folliculitis or inflamed hair follicle, in particular is a pimple like lesion with associated itching or pain. It usually resolve on its own and has a tendency to recur. It is important to keep the area clean and dry. Warm compress may also help. Avoid manipulating the lesion to avoid secondary infection.
If it still persists, it would be best to have it evaluated by your doctor or have a referral to a colorectal surgeon to rule out rectal causes like fistula or fissures.
Take care and keep us posted.