It is an irritation and inflammation that occurs in the lining in the beginning of the small intestine, which is called the duodenum. The duodenum is connected to your stomach and is a tube that is about twelve inches long and curves into a C-shape. The far end of this tube blends into the rest of your small intestine. This is all part of the digestion system and is what connects your stomach to the rest of this system. This medical condition is similar to gastritis, which is inflammation of the lining of your stomach. Most of the time, these two medical conditions occur together. There are different types of duodenitis. Many times this will lead to the formation of duodenal ulcers, which are open sores in the lining of this tube.