It COULD BE an infection of the hair follicle in the perianal buttock area.. or could be a pilonidal cyst like you are presuming.. we need to examine and see if you might have to see a surgeon for draining the pus collection in the area. Also need to know if there is coexisting diabetes in the patient and need to know history of any drug allergies. I would suggest to get these details cleared up with a doctor before starting the treatment. You should send me images through private online consult for a better assessment of the problem and approaching it accordingly. Hope this is helpful.