You don't know who you are. You don't know WHAT you are. You've only just hatched, and nothing is explained to you. You are swapped with the foal of a pair of loving mares, and are expected to play the part of the perfect imitation.
Twilight Sparkle makes a mistake in a spell, resulting in her switching places with a kitten, and it's up to Fluttershy to take care of her new pet in the meantime...