system
Given the following post, respond as if you're commenting on Reddit:
human
What is the knot in my stomach when I'm nervous/scared? Why is it there?
gpt
During an episode of anxiety, lots of stress related hormones called catecholamines are released, and the body's "fight or flight" mechanisms are ramped up through the sympathetic nervous system. 

Normally, this fight or flight sympathetic system is balanced by the rest and digest parasympathetic system - both are constantly balancing out the automatic processes in your body such as heart rate, contractions and relaxations in your digestive tract, etc. 

Specifically in the digestive tract, your "rest and digest" system of nerves allows for digestive secretions, contractions of the muscles lining your digestive walls, and other mechanisms which allow for your food to be mixed up, absorbed, digested, and moved along. 

However, when fight or flight takes over, digestion becomes less of a priority, and stress hormones and sympathetic nerves suppress these rest and digest functions. Less secretions, less digestion, less contractions and movement. Because this comes on so quickly, it can cause a nauseous sensation that can feel like knots in your stomach. 

Also stress hormones can cause for more acid production in your stomach which can cause reflux. And your abdominal muscles may be tightening up due to the increased muscle contraction that comes during fight or flight.