Problem with dress sizing....suggestions?

Ok so I tried my dress on today and relized it was a bit big up top at first I couldn't figure out why since I haven't lost weight but then I remembered I had been breast feeding at the time of final fitting and didn't even think about my boobs getting smaller afterwards so now im stratus I won't have time for alterations, wedding in 4 wks, do u think maybe a really good pushup bra would work or what can ido,( I don't want a water bra) and do I wear it under mycorset or get another one just for mywaist