Her menstrual cycle will tend to make her breasts swell the week prior and the week during menstruation. Also, increased sexual activity sometimes tends to cause swelling (especially if there is a lot of stimuli to her breasts).

Typically after her cycle ends, the swelling should go down. That is, if her breasts aren't receiving an intense amount of "attention". lol I know mine become about one cup size larger, and are very sensitive when Sir gives them a good amount of His attention.