It's probably hormonal. I'm not sure why hormonal shifts occur, but you might want to consider buying organic meats.
Try primrose oil pills for a few weeks if it continues.
The various causes of breast soreness are premenstrual syndrome, hormone therapy, pregnancy and breast feeding, fibrocystic disease, use of oral contraceptives and rarely a breast malignancy. Please consult a gynecologist and discuss these possibilities.
In absence of pregnancy, it can be due to hormonal imbalance and should improve with time. Apply warm compresses and take some anti inflammatory pain killers. If it persists then get it evaluated from a gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.