Hormonal Changes Post-Menstruation: This is the most likely diagnosis due to the timing of the symptoms in relation to the menstrual cycle. Hormonal fluctuations after menstruation can cause breast tenderness and changes in nipple sensation.

Mastitis: An infection of the breast tissue, which can cause pain, swelling, and hardness, especially if the patient is breastfeeding or has recently stopped. However, mastitis is less likely if there's no history of breastfeeding or recent weaning.
Duct Ectasia: A condition where the milk ducts in the breast become widened, leading to symptoms like nipple discharge, inversion, or pain. It's more common in women nearing menopause but can occur in younger women.
Montgomery's Tubercles: Small bumps on the areola that can become more pronounced during hormonal changes, such as those experienced during the menstrual cycle or pregnancy. They can sometimes cause discomfort or be mistaken for hard nipples.

Breast Cancer: Although less common in younger women, breast cancer can present with changes in the nipple, including hardness, retraction, or discharge. It's crucial to rule out cancer due to its severe implications.
Paget's Disease of the Breast: A rare form of breast cancer that affects the skin of the nipple and, sometimes, the areola. Symptoms can include itching, redness, and nipple discharge, but hardness could be a presenting symptom in some cases.

Periductal Mastitis: A rare condition that causes inflammation of the ducts under the nipple, leading to symptoms like nipple retraction, discharge, and sometimes hardness or pain.
Zuska-Atkinson Syndrome: A rare condition characterized by chronic inflammation of the areola and nipple, leading to symptoms like hardness, retraction, or inversion of the nipple. It's often associated with smoking.