Breast aches and pains are cause by compression of the nerve endings in the breast. Basically, neurotransmitters in the nerves send messages to the brain that the breast hurts. Anything that causes the nerves to be compressed can cause breast pain, including a breast mass, breast cyst, fluid/inflammation (which can be caused by infection or trauma), and scarring.
A poorly fitted bra can also cause breast pain because it compresses areas of the breast, and fluid may be trapped in certain locations that compresses the nerve endings. Not wearing a good supportive bra while running or engaging in high-impact exercises can cause trauma to the chest wall and breast. Chest wall tenderness can last for several weeks because the muscle is strained. If you experience any of these symptoms, talk to your doctor to be treated.
Iron deficiency can also cause breast pain, due to iron's function of regulating thyroid hormones. There have been studies showing that 6 mg of an iodine supplement can reduce breast pain and boost thyroid function. If you have breast pain and think it is caused by iodine deficiency, you should discuss with your doctor to determine whether iodine supplements are best for you.