It’s not that a doctor wous say, “oh that’s stomach cancer.” It’s that a doctor would pursue determining what the matter was.
Maybe, *if* the symptoms present as severe. Otherwise, there are many things it *might* be, many of which are temporary and not severe. This is one of the major reasons that liver, pancreas, colon, kidney and stomach cancers kill, is by the time anyone realizes what they are, it is too late.
This means that doctors have to use discriminators to narrow down what a disease that *is* presenting serious symptoms might be.
Age windows. Had she been older than 50, they would have looked for cancer, because the vast majority of cancers of these types happen after 50.
Had members of her family had stomach or other forms of cancer, this is also a big indicator. Japanese doctors are always on the look out for stomach cancer, because the Japanese eat a lot of smoked fish, so stomach cancer is common.
But cancer is rare in teens. And the most common of those rare cancers is bone cancers, leukemia, brain tumors and lymphoma. Stomach cancer in teen girls is extremely rare.