Breasts are milk glands and all breasts contain some degree of lumps (or "cysts”). Some breasts have more lumps than others, usually due to hormonal changes; these lumps may cause breast discomfort and pain on compression
Oestrogen, progesterone, and prolactin cause cells in the breast tissue to swell. This is a normal hormonal change during menstruation. Symptoms of fibrocystic breasts are the most intense before menstruation then usually leave immediately after.
An estimated 30 to 60 percent of women experience fibrocystic breast pain.