Gastritis is inflammation of the lining of the stomach. It may occur as a short episode or may be of a long duration. The most common symptom is upper abdominal pain. Other symptoms include nausea and vomiting, bloating, loss of appetite, and heart burn. Others may have no symptoms. Complications may include bleeding, stomach ulcers, and stomach tumors. When due to autoimmune problems, low red blood cells due to not enough vitamin B12 may occur, a condition known as pernicious anemia.