肚子饿 = hungry (belly is hungry), 肚子疼 = stomachache, 拉肚子 (lā dùzi) = to have diarrhea. These are the most common collocations.
mistakes
肚子 (dùzi) refers to the belly/abdomen area. The actual digestive organ 'stomach' is 胃 (wèi). But colloquially, Chinese people say 肚子疼 even when they mean their stomach hurts.
