Gaining weight solely in your stomach may be the result of specific lifestyle choices. The two S's — stress and sugar — play a significant role in the size of your midsection. Certain medical conditions and hormonal changes can contribute to abdominal weight gain.
It might be as simple as eating too much too fast, or you could have a food intolerance or other condition that causes gas and digestive contents to build up. Your menstrual cycle is another common cause of temporary bloating. Sometimes a bloated stomach can indicate a more serious medical condition.