Butterflies in the stomach is a phenomenon characterized by the physical sensation of a “fluttery” feeling in the stomach. This sensation can be a physical sensation related to the body’s fight or flight response or it can be an ineffable experience related to the psychology of love or nervousness and excitement.
Some believe that this is caused by the release of adrenaline when one is nervous, which pulls blood away from the stomach and sends it to the muscles. This reduced blood flow, in turn, causes the stomach to temporarily shut down, and is possibly the reason for reduced appetite during love sickness. -wikipedia.org