get butterflies in (one's) stomach

To have a feeling of nervousness, often before a performance or undertaking of some kind. I always get butterflies in my stomach before I go on stage and perform. I know you've got butterflies in your stomach, but you have to tell Sarah how you feel about her! Even though I'm student council president, I always get butterflies in my stomach before I have to make announcements in front of the whole school.

get/have ˈbutterflies (in your stomach)

(informal) get/have a nervous feeling in your stomach before doing something: I always get butterflies (in my stomach) before an interview.