Urinary Tract Infection (UTI) with systemic symptoms: Although the primary symptoms are gastrointestinal, a UTI can cause systemic symptoms such as nausea and vomiting, especially if the infection has spread to the kidneys (pyelonephritis).

Gastroenteritis: An infection of the stomach and intestines that can cause vomiting, nausea, and other gastrointestinal symptoms. It's possible that the UTI symptoms are coincidental or not yet apparent.
Appendicitis: Inflammation of the appendix can cause nausea, vomiting, and abdominal pain. Although the primary symptom of appendicitis is usually abdominal pain, it can sometimes present with gastrointestinal symptoms.
Food Poisoning: Consuming contaminated food can cause vomiting, nausea, and other gastrointestinal symptoms.

Sepsis: A life-threatening condition that occurs when the body's response to an infection becomes uncontrolled and causes widespread inflammation. A UTI can lead to sepsis if not treated promptly, and gastrointestinal symptoms can be an early sign.
Kidney Stone: A blockage in the urinary tract can cause severe pain, nausea, and vomiting. Although less common in adolescents, it's a critical diagnosis not to miss.
Pyclonephritis: An infection of the kidney that can cause systemic symptoms, including gastrointestinal symptoms.

Hemolytic Uremic Syndrome (HUS): A rare condition that can occur after a UTI, characterized by anemia, acute kidney failure, and low platelet count. It's more common in children under 5, but can occur in adolescents.
Tuberculosis (TB): A rare cause of UTI, especially in immunocompromised individuals. TB can cause a wide range of symptoms, including gastrointestinal symptoms.