Hello I share your concern Looking at your description it might be related to infection, Local allergy or an injury or inflammation during anal sex I would like more information.to help you better like Since how long you have this problem? Relevant medical history like diabetes or other metabolic disorder ? In my opinion you should consult a dermatologist for physical examination and series of investigations to establish a diagnosis like urine culture and sensitivity test and routine blood profile. You may require a course of antibiotics under supervision of your doctor Meanwhile have plenty of fluids Maintain good hygiene Avoid sex or use condom until it heals completely Thanks for your question Wish you good health