Appendicitis is when your appendix becomes sore, swollen, and diseased. It is a medical emergency. You must seek care right away. It happens when the inside of your appendix gets filled with something that causes it to swell, such as mucus, stool, or parasites.Appendicitis typically starts with a pain in the middle of your tummy (abdomen) that may come and go. feeling sick (nausea) being sick. loss of appetite. constipation or diarrhoea. a high temperature