A stomachache is a pain in your stomach or the area around it. In real use, English speakers use it loosely for almost any pain in the middle of the body — from eating too much, from bad food, from period pain, or from nerves before an exam. Doctors may be more exact, but ordinary people are not. Two grammar points matter more than the meaning here. First, in American English it is usually written as one word, stomachache; in British English you often see two words, stomach ache. Both are correct. Second, and this catches everyone: it takes 'a'. You say 'I have a stomachache', not 'I have stomachache' — though British speakers do sometimes drop the 'a', which is why students see both and get confused. Follow the 'a'. It is an everyday word, fine to say to a friend, a teacher, or a doctor. It usually means something mild and temporary; for something serious, people say 'stomach pains' or name the real problem.