(For My Little pony Friendship is Magic fan ONLY) What is the Dark Secret?

So, there is an episode coming out that's called A Canterlot Wedding. It will have a new Alicorn named Princess Cadence(shown in picture). In something I read about the upcoming episode, I read that Twilight Sparkle will discover a dark secret about Princess Cadence. What do te think the dark secret is?



(only fan of My Little pony Friendship is Magic will understand what in the world I'm talking about)