Stress, fear, and anxiety causes an overactive adrenaline response, also known as the fight or flight response. An overactive adrenaline response is when your adrenal glands release adrenaline into the bloodstream after the fight or flight response is activated. This adrenaline then affects different body systems and organs, and is responsible for the physical symptoms of anxiety, including increased heartbeat, sweating, trembling, muscle tension, and butterflies in the stomach.