You should get examined by a Surgeon to know the type of swelling it might be an enlarged lymph node and may be due to some infection or a benign tumor of breast(eg Fibroadenoma)that is related to menstrual cycle like this becomes painful.
You may need examination by an expert and if needed USG, Mammography etc and if the swelling is a concern please get a FNAC done to arrive at tissued diagnosis for better management.
Fibroadenoma being a benign breast lesion can be treated easily and may require removal(surgically).