system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening inside the stomach when you feel sick?
gpt
Your body has the ability to make itself sick. If you eat something and the body wants to get rid of it, it can secrete a substance that will induce vomiting. Once you vomit, it removes the irritant, and it removes the vomit inducing substance and this is why you usually feel better almost instantly. 

Think of it as a natural syrup of ipecac courtesy of your stomach, liver, and gallbladder. 


Edit*** To answer peoples questions, a chemical called TNF (tumor necrosis factor) stimulates the brain to shut down the digestive system by relaxing the stomach and digestive tract. This slows digestion to prevent absorbing the irritant further. TNF is produced by several different cell types in the body and has links to anti nausea medications and cannabanoids as a method of reducing nausea by blocking TNF receptors. Any medical researchers out there, feel free to correct me. 

The mucus lining of the stomach can become irritated, and this is what causes the actual contractions that result in vomiting.