Hi! Good morning. I am Dr Shareef answering your query. From the history, it seems that you had a benign cyst/lump at the site, which got inflamed by the repetitive irritation caused by the bra strap over there. One of such common swellings on the back is a sebaceous cyst , which got infected in your case. Because of inflammation/infection, it became red, painful and tender. If I were your doctor, I would prescribe you with a broad spectrum antibiotic, an anti inflammatory drug along with a proton pump inhibitor. If not relieved, and in case it turns out to be an abscess, I would refer you to a general surgeon/ER room in your area for further management. I hope this information would help you in discussing with your family physician/treating doctor in further management of your problem. Please do not hesitate to ask in case of any further doubts. Thanks for choosing health care magic to clear doubts on your health problems. I wish you an early recovery. Dr Shareef.