What causes sore breasts can be other than pregnancy or period?

Mastalgia: Reasons for painful breasts (bilateral) can include: cyclic pain before the period, stress, medication changes. Fluctuating hormone levels & pregnancy. Pain in one breast could be related to a cyst or a fibroadenoma. Breast cancer would not be a frequent reason for breast pain. Exercise with a bra that is lacking in support is also a possible cause.