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