Best Answer:  Butterflies in the stomach is a medical condition 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 true love or nervousness.

Some believe that this is caused by the release of epinephrine (adrenaline) when one is nervous, pulling blood away from the stomach and sending it to the muscles. This reduced blood flow, in turn, causes the stomach to temporarily shut down, and possibly the reason for reduced appetite during love sickness

It's when you're so excited your tummy feels sensitive and you feel almost ill. In itself it is not a nice feeling but as it is usually caused by excitement, most people enjoy it.

I believe it is caused by your increased heart rate and reduced blood flow to your abdominal area as your brain is often working faster and you may shake or sweat.