Breast lump or lump in the breast is a common condition but may be a sign of breast cancer. Breast tissue also extends into the armpit. If you think you have a breast lump, always have it examined by a doctor. Breast infection refers to an infection within the breast tissue. Also known as mastitis, it can develop in many breastfeeding women. A breast abscess is a painful collection of pus that forms in the breast. Most abscesses develop from a bacterial infection.