What can cause a constant hard distended stomach?

Lots of things: Distension after meals suggests gastroparesis (a slow stomach) or dysmotility of the small or large intestine. Distension is often reported by irritable bowel syndrome patients who exhibit no structural pathology. Partial or complete obstruction of the bowel can cause distenstion. Accumulation of ascites fluid (benign or malignant causes), pregnancy, organ enlargement, herniation are others.