"I am Big." "I a girl!"

The past few days whenever I try to help Laney with something. She says to me "Mommy I a girl!", meaning she is able to do it herself. After she said this to me half a dozen times in one day I said at one point, "Laney Bug I know you are a big girl." and she answered me, looking rather annoyed "Mommy I am big and I a girl, not a big girl." Hmmm... I wonder how her three year old mind differentiates the two statements.