Hello,
Welcome to ehealthforum and thanks for writing
Bruise in the breast can be due to any injury,inflammation,allergic reaction,trauma,hormonal changes etc.You can take some OTC available pain killers as Ibuprofen for pain relief.Applying ice pack wrapped in cloth might also help.
Observe for three days and it might subside but if the symptoms persist or there is development of other symptoms as lump,swelling,fever then consider visiting your healthcare provider for physical examination.Relax and avoid any stress.
Take care!