Hello, You are on antibiotics due to mastitis, presently you are feeling pain in the back while taking deep breaths. It should not be due to the antibiotic unless you have sulfa allergy. If you have sulfa allergy report urgently to your treating doctor & stop taking the medicine, it should be replaced by other medicine suits you better. Get a follow up treatment done to rule out any developing abscess or sepsis(rare) that may cause you such symptoms. However, mid back pain can also be due to issues with the spine, slip discs, spinal stenosis, tumors, muscle strain, overuse, poor posture, etc. Maintain correct posture when sitting, walking or sleeping. Take acetaminophen, one sos after food for relieve. You are suggested to get clinically examined by a GP, get all investigations done. Get all required Imaging tests done to detect the cause of your discomfort. You will be accordingly treated. Hope I have answered your queries. Let me know if I can assist you further. Take care Regards, Dr. Nupur K