During your period, the breasts reabsorb the extra fluid instead of discharging it. Often in the years before menopause the hormones are a bit off and there may be more breast tenderness. This type of cyclic breast pain occurs in both breasts, sometimes more one sided than another, and it is often felt in lateral aspects where there is more breast tissue. It may differ from month to month but always gets worse before a period then eases with menstruation. A diet high in salt increases swelling by causing fluid retention, which also puts a strain on breast tissue.
Some recent studies reveal that fatty foods, mostly animal fats, also contribute to breast pain.
The reason is less clear but it may have to do with the animals own hormone and their diet. Again the reason is unclear. We are only beginning to explore the links between the immune system, emotions, and our hormones. Some medications can cause or contribute to breast pain. Hormones, either birth control pills or hormone replacement therapy.