Ever since Discord promised to be a good guy he has become very bored. The once mighty embodiment of chaos is now reduced to a trickster who plays simple yet reality altering pranks, To put it simply he is bored out of his skull.
After a visit to Fluttershy, he decides to try to help her to be less afraid of everything.
What started as a small joke ends up in catastrophe, in his sleep he accidentally cast a curse upon Fluttershy.
So now whenever she get scared she shrinks in size.
Hopefully Discord will be able to cure her before she becomes so tiny that she no longer exists in the same universe as him. But fate has other plans. And Fluttershy is not the only one shrinking.