Are you looking for idioms for the experience of shock or for describing a person who is not who she says she is? I can't think of any that captures the feeling of shock at finding out someone is not who you thought they were (although there may be some), but here are some idioms for shock:
It threw me for a loop (more mild surprise)
I was floored
My jaw dropped
At a loss for words
For more idioms about a person, another one is "true colors." So, you could say, for example, "I was floored when he showed me his true colors."