Swelling is a common symptom of breast cancer, especially in large tumors and inflammatory breast cancer. It is caused by a build-up of fluid in the interstitial tissue which is usually the result of a blockage to the venous or lymphatic return. In other rare cases, an inflammatory type of cancer is characterized by swelling, redness, and other inflammatory changes to the breast tissue. It is also painful and tender to the touch.
In some cases, swelling is severe enough to cause a visible change in size when comparing both breasts. In these cases, the skin feels tight and stretched, and it is often associated with breast pain. Thus, if you feel changes in your breast size or swelling that is not associated with your menstrual cycle, talk to your doctor right away to investigate its cause.