Fibrocystic Changes: This is the most common cause of mastalgia in women, especially in the 50-year-old age group. Fibrocystic changes are characterized by lumpy or rope-like breast tissue, which can cause cyclic or non-cyclic mastalgia. The absence of abnormalities on mammogram or ultrasound supports this diagnosis, as fibrocystic changes are often not visible on imaging.

Hormonal Fluctuations: Hormonal changes, especially in perimenopausal women, can lead to mastalgia. This could be due to fluctuations in estrogen and progesterone levels.
Breast Cysts: Although the ultrasound did not show any abnormalities, small breast cysts can sometimes cause mastalgia and may not be visible on imaging.
Mastitis or Infection: Infection or inflammation of the breast tissue can cause mastalgia, although this is less likely in the absence of other symptoms like redness, swelling, or fever.

Inflammatory Breast Cancer: Although rare, inflammatory breast cancer can present with mastalgia and skin changes, but it may not be visible on mammogram or ultrasound in its early stages. It's crucial to consider this diagnosis due to its aggressive nature and poor prognosis if missed.
Breast Cancer with Negative Imaging: Some breast cancers, especially those that are invasive or have a dense breast tissue background, might not be visible on mammogram or ultrasound. Clinical correlation and possibly further imaging or biopsy might be necessary.

Sarcoidosis of the Breast: Sarcoidosis is a systemic disease that can rarely affect the breast, causing mastalgia among other symptoms.
Neurogenic Pain: Pain referred from the chest wall or intercostal nerves can sometimes be perceived as mastalgia. This could be due to conditions like costochondritis or thoracic outlet syndrome.
Thyroid Disease: Some thyroid disorders, particularly hypothyroidism, have been associated with mastalgia, although this is less common.