have butterflies in (one's) stomach
To have a feeling of nervousness, often before a performance or undertaking of some kind. I almost didn't go on stage and perform tonight because I had butterflies in my stomach. I know you have butterflies in your stomach, but you have to tell Sarah how you feel about her! Even though I'm student council president, I always have butterflies in my stomach before I make announcements in front of the whole school.
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