^ Queenie makes an excellent point. Don't worry about fitting into that dress. Imagine, if guys did this (I know some do already, but imagine if someone---say, me---did that):
AMMMMMMMIIIIIIIIIII!!!! Where's my shirt, this one doesn't fit me anymore. I think it shrunk.
As you can see, not a pretty picture.
It's not impossible.
Someone close to me lost 22 pounds in a month once and not in a very healthy way. She had only 2 bites of an apple in 5 hour intervals (finishing half an apple a day), took prescription diet pills from a highly unreliable "nutritionist" (as they all are, here), and over-exercised (caused a scene at the gym a few times as she fainted a few times). After losing all that wieght, she managed to get herself admitted in the hospital for dehydration and blood poisoning and then gained all that weight back, and more. Not good.