The various causes of breast
soreness are premenstrual syndrome, hormone therapy, pregnancy
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.