One common sign of nervousness is a "butterflies in the stomach" feeling. This is caused by adrenaline, which is pumped into our bloodstream when we are excited and alert, or feeling troubled about our situation. Some people feel nauseous or even vomit when they experience these feelings. Anxiety can also cause sweating, an accelerated heartbeat, and difficulty breathing – all of which are natural reactions to danger that help prepare the body for action.