Where does the "butterfly" sensation come from?

Question

in your stomach, why do you feel that way? like how does it happen?

Answer 1

The body has a number of different systems, but only enough blood to run a few of them at full power.  After you eat a large meal, the body needs to crank up the power of the digestive system to break down food and absorb nutrients.  That's why you feel tired after a large meal.  When you need to physically exert yourself, your body diverts the blood flow (with it's nutrients and oxygen) to the muscles.

When you get nervous, your body anticipates a fight or flight situation.  It diverts blood away from the digestive system and to the muscles.  If your digestive system throttles down while you have food in your stomach, it can cause nausea (which is the real reason you don't want to go swimming shortly after you eat).

You get butterflies in your stomach because the part of your brain that deals with this sort of thing is very primitive.  It evolved many millions of years ago, so all it's geared for is "fight or flight."  You're probably not in danger of being eaten by a dinosaur while you're giving a presentation in front of your class... but the primitive, instinctive part of your brain doesn't know this.  All it knows is "there might be danger, so I have to be ready to fight or run."

Answer 2

It is probably due to a curculatory phenomenon which happens in extreme emotional states.  Blood is drawn from the lining of the stomach by constriction of the vessels and dispersed more into the large muscle groups of the body.  This is a survival trait where a potentially life-threatening condition would give an advantage to individuals whose bodies could divert blood to regions of the body that would aid in fight or flight, like legs and arms.