I don't know Mandarin but I notice that you do. So I can help by explaining the meaning of the term to you and you can find out the right phrase yourself.
"Butterflies in the stomach" is an English phrase that refers to having a feeling of nervousness, anxiety or the "fight or flight" response. This is the fluttery feeling we all get in our stomachs when we sense impending and fast approaching danger or are anxious about something.
See this link for more information : "http://en.wikipedia.org/wiki/Butterflies_in_the_stomach"