Hello,
Antibiotics can predispose to fungal infections. Hence fungal infection of the butt area should be ruled out. Most cases respond to over-the-counter products, which contain any of several basic ingredients: miconazole, tolnaftate, terbinafine,ketaconazole and clotrimazole.You can consult a pharmacist and get any cream or lotion containing these ingredients.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
May be dusting of clotrimazole powder be of help, if there is a fungal element here.