Penny wants to tell Amy the truth, because she thinks that someone will say something about it to her and it should be them now instead of someone later at the actual wedding.
Bernadette originally disagrees, but eventually caves and goes with Penny to tell Amy the truth.
Bernadette and Penny agree that it’s beautiful and perfect, so Amy buys the dress and is a happy woman.
As soon as Penny brings up the topic with Amy, though, and she moans, “What’s wrong with my dress?”