The phrase "butterflies in your stomach" refers to a bodily reaction to stress in which an individual experiences a fluttering feeling in her stomach due to chemical changes in the blood stream. The feeling is often accompanied with nausea, increased heart rate, dry mouth, and shaky or damp hands.
The feeling of "butterflies in your stomach" is attributed to the "fight or flight" response where the human body prepares to either face or flee from a stressful situation. In these moments, the body releases adrenaline and other hormones; the former increases heart rate and blood pressure, which in turn improve circulation in the body's muscles and respiratory system. At the same time, this reaction limits the amount of blood reaching non-essential organs such as the stomach. These bodily changes result in the feeling commonly referred to as "butterflies in your stomach."