Hello, The combination of shoulder pain and a lump on the back of your shoulder that has discharged pus may indicate an infection or abscess. Abscesses are typically caused by a bacterial infection and can cause pain and swelling in the affected area. Treatment may involve antibiotics and/or drainage of the abscess. In some cases, surgery may be necessary. So, I recommend to get it checked by your caring doctor as might require prescription medications. Meanwhile, keep the area well clean, apply betaine and antibiotic ointment locally until you see your caring doctor. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Albana Sejdini, General & Family Physician