It is uncommon to experience increasing pain and noted swelling in one of the breasts several months after the surgery. This could be suggestive of an early seroma possibly. If this continues to expand, be sure that your Plastic Surgeon follows you carefully. A mammogram/sonogram may be necessary. If surgery is required, be sure that the fluid removed is sent for culture analysis and cell pathology review. This would help to eliminate any concern of infection or cancer diagnosis( very uncommon, I must add).
You may have simply bruised the breast because of some physical activity. Initially cool compresses and antiinflammatory medications may be suitable to try to limit this swelling.
Good luck for the future.
Frank Rieger M.D. Tampa Plastic Surgeon