You may also have diarrhea or a fever. It is sometimes called stomach flu, but it is not the flu. Germs like viruses and bacteria can cause it. You can catch it from someone else who has it, or you can get it from food poisoning. Food poisoning can happen if you eat foods that contain harmful germs.
The circular muscle located at the entrance and exit to the stomach is called the sphincter. So for this question, you are talking about two distinct sphincters. This anatomical structure is able to open, and contract or close as needed.
the distal aperture of the stomach or abomasum, opening into the duodenum. The term pylorus is variously used to mean the pyloric part of the stomach, and the pyloric antrum, canal, opening or sphincter. A ring of muscles, the pyloric sphincter, serves as a 'gate', closing the opening from the stomach to the intestine.