“Butterflies in the stomach is a phenomenon characterized by the physical sensation of a "fluttery" feeling in the stomach.
1But·ter·fly /ˈbʌtɚˌflaɪ/ noun
plural but·ter·flies
butterflies [plural] informal : a nervous feeling in your stomach
▪ Even experienced musicians sometimes get butterflies before a performance. ▪ I have butterflies in my stomach.
This is a physical sensation related to the psychology of lovestruck, love sickness, nervousness and excitement.
The idiom "I have butterflies in my stomach" referred in many chic literatures as an expression when one person falls in love with others.
I, myself, periodically experienced it before every BISELMOL (Biology cells and moleculars) examination and also everytime I have to do something important in life. Think about it lately, why is falling in love cause us the same sensation as getting biselmol exam ?