It is a reference to the fairy tale of Cinderella. Actually, it refers to the Disney animated cartoon. A pumpkin is kind of gourd. It can be hollowed out. In the cartoon, the fairy godmother magically turns Cinderella's rags into a beautiful gown. She turns a pumpkin into a fancy coach. She warns Cinderella that when the clock strikes midnight, the magic will work off. The coach will turn back into a pumpkin. Midnight is a deadline. Cinderella knows she has to leave by midnight. Young women who need to get back home by midnight (or some other time) will joke about it. To make it funnier, instead of saying "my coach will turn into a pumpkin," they say "_I_ will turn into a pumpkin." In the phrase you cite, the woman is saying "I am already a pumpkin." That means it is already past midnight. She has already been out too late.

A real pumpkin: http://cdn.sheknows.com/articles/2012/10/isolated-pumpkin.jpg Cinderella's pumpkin coach: http://tinyurl.com/ov2tblr