Hello, Welcome to healthcare magic. From your history and description,it seems to me that you had a sebaceous cyst which has now got infected. Sebaceous cyst appears as a painless, cystic mass usually on the face and trunk and may not cause much discomfort. It has a tendency to get secondarily infected, in which case it becomes red, swollen and discharges pus as in your case. You will benefit from taking a course of oral antibiotic like penicillin/ cephalexin for 5 days. You could also use a topical antibiotic cream with 2% mupirocin twice a day for 7 days. Once the infection has subsided, You need to get the cyst excised by a dermatosurgeon. Hope this helps you. Take care.