breast swelling

Puberty - causes breast size increase.
Menstrual cycle - may change breast size during the cycle.
Contraceptive pill - may cause slight breast size increase.
Hormone replacement therapy - may cause slight breast size increase.
Mastitis
Gynecomastia
Breast abscess
Sexual arousal - causes breasts to swell
Lymphatic obstruction - may cause one swollen breast
Pregnancy
Newborn infant breast swelling - some newborns get temporary breast swelling from maternal hormones.
See also causes of breast lumps, swelling, and breast pain