You are a tiny. You were the partner of Kitty but you accidentally shrunk and Kitty may have accidentally freaked out when she saw you tiny and this sends you toppling into Carrie's shoe when she accidentally drops you inside of it. This is when Carrie comes and inserts her foot in to wear it like normal. Carrie's toes and foot come at you like a freight train and Kitty can only watch in horror as she anticipates what you'd do to survive the inside of Carrie's shoe