(For My Little pony Friendship is Magic fans 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 u think the dark secret is?



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