Rare things are rare. A person can never tell if he is the rare unlucky one to get cancer at a young age. More importantly, a person cannot live his life worrying about rare bad events, like getting struck by lightning. So, the best one can do is live a healthy life, see the doctor for evaluation, and then every month for follow-up (if the patient or dr. Continue to worry). Loose stools cannot cause stomach cancer.
Very unusual. Loose stools do not cause stomach cancer. Additionally, this would be an uncommon symptom of stomach cancer. Stomach cancer in the 20's is also very rare, but not impossible. I have unfortunately taken care of a few patients this young. It is usually related to a genetic mutation or a heritable syndrome. See your doctor for details.