Right after the events of the 'Changeling invasion', Fluttershy felt something wrong when the shield spell was used. Her true form exposed: a changeling hybrid...

I just wanna note that this was written way before a lot of big reveals and other stuff. I'd consider this more of an alternate universe of pre-season 4 stuff. I've been away from the fandom for so long that I have no clue on the new stuff.