the.com/butterflies in the stomach

means The fluttery, swoopy, tight feeling you get in your stomach when you're nervous or excited — classically about love, but also useful for stage fright and job interviews.

from A literal little word-picture, first recorded in American English in the early-to-mid 1900s: anxiety or excitement diverts blood flow and rattles the gut, and someone described the sensation as feeling like insects loose in there, wings brushing the stomach walls. 'Butterflies' specifically (rather than just 'bugs') seems to have stuck because the feeling is fluttery and light rather than crawling or heavy — it suggested something delicate and agitated at once. No single coiner is known; it's one of those plain, sensory phrases that spread because it simply described the feeling too well to need inventing twice.