Breasts often swell and become more tender due the post scarcity effect during which the breasts soak up large amounts of iodine. This appears to be an evolutionary strategy for the body to hoard nutrients during times of famine. Once your body realizes the Iodine bounty is permanent, the breasts will adjust and the pain and swelling will disappear.
This phenomenon has been reported many times. The thyroid may also swell temporarily.