The most common cause of breast lumps is fibrocystic breast disease. In this illness, one or more fluid-filled masses form as cysts, causing recurrent flare-ups of tender lumps in one or both breasts, especially before menstruation. Breast cysts are often linked to premenstrual syndrome (PMS). The lumps are movable, and they fluctuate in size during the menstrual cycle. Breast cysts most commonly occur in women between the ages of thirty and fifty.