What could cause hives on the stomach?

Many things: Unfortunately, the location of hives offers little diagnostic information. Assuming this is new for your, the most common cause is infection (often very minor in nature) resulting in activation of innate immune pathways in skin that trigger mast cells. Usually, hives resolve and we never know the cause. For other causes: http://emedicine.medscape.com/article/137362-overview#aw2aab6b2b3aa