Knowing how your breasts normally look and feel is an important part of breast health. Finding any breast issues as early as possible gives you a better chance of successful treatment. 

As you know, breasts are very responsive to hormones in the menstrual cycle or in HRT. Hormones called estrogens increase before a period, causing milk ducts and glands to swell. This can trap fluid in the breasts and cause swelling and lumpiness before your period is about to arrive. Later when hormonal levels even out, the breast swelling goes away on its own. 

Possible reasons for breast swelling

• Puberty
• PMS (both breasts)
• Pregnancy (both breasts)
• Lactation (both breasts)
• Breast infection (mastitis - inflammation of the breast tissue or breast abscess). 
• Lymphedema (the build-up of lymphatic fluid in tissues)
• Cancer

Swelling of all or part of a breast (even if no distinct lump is felt) can be a sign of breast cancer. A combination of breast heaviness, burning, aching, and an increase in breast size and tenderness can signal inflammatory breast cancer. Sometimes breast cancer can spread to lymph nodes under the arm or around the collar bone and cause a lump or swelling there, even before the original tumor in the breast is large enough to be felt. Swollen lymph nodes should also be checked by a healthcare provider.

Although any of these symptoms can be caused by things other than breast cancer, if you notice any of them, they should be reported to a health care professional so that the cause can be found.