It was recorded in more than 2000 years ago in a TCM classic, Huang Di Nei Jing (Yellow Emperor's Canon of Internal Medicine), that women have their menstrual cycles from the age of 14 until age of 49.
Xue Zheng Lun (Blood Disease Theory), written by Rong Chuan Tang in Qing dynasty, explained that the nutrients from food would be transformed by Spleen and send upward to breasts to become breast milk, then goes down to uterus to become menstrual blood. This means the fullness and tenderness in the breasts before period is because they are filled with breast milk. If the woman is not pregnant, breast milk is no longer needed. Before period comes, the Heart will pump the breast milk down to the uterus along Ren and Chong meridian and become menstrual blood.
This may sound like nonsense to anyone with a little bit of modern medicine knowledge; however, this is a phenomenon clinically observed by ancient Chinese. Based on this phenomenon, they formed a hypothosis and observe what happened when breast milk didn't descend to uterus and discharge completely.
They found if unused breast milk stayed inside the body, it became toxins and cause various diseases. Just like the food we eat turns into feces in the G.I tract. If there isn't a regular bowel movement, it can create a health problem.
If breast milk remains in the breast, it can become breast lumps (also called proliferative breast disease) in a mild case, or breast rocks (now breast cancer) in a severe case;
if milk goes to lymphatic system, it can become lymphoma whether benign or malignant;
if milk goes to Lung, it can become Lung tumors;
if milk goes to Liver, it can become Lv tumors;
if milk goes to Heart, it can become Lupus;
if milk goes to the spine/marrow, it can present as Leukemia;
You get the idea. So go see your TCM practitioner or acupuncturist to regulate menstruation without chemical pills or artificial hormone before it turns into anything serious. Prevention is the best cure.