“I’ve got butterflies in my stomach” – this relates to more than just that feeling you get when you’re around someone you’d like to kiss, as the same feeling also comes when you’re nervous (eg before an exam or the first day on a new job).
These feelings are most likely related to the fight or flight reflex. When something stressful is about to happen (in evolution this would be fighting a predator/rival), the body switches into an ‘action mode’, which sends adrenaline speeding through your bloodstream. In addition, the autonomic nervous system diverts blood away from non-essential systems (such as your stomach and intestines) and sends it to the muscles and brain.
The vagus nerve, which is a key mediator of this effect, slows stomach function, as well as increasing the heart rate. This may be the reason that your stomach, which doesn’t need to be active during these situations, reports a different sensation to your brain.
Plus, take a look at: