Gastroesophageal Reflux Disease (GERD): The symptoms of nausea, vomiting, and a sensation of a knot in the upper stomach could be indicative of GERD, especially if the patient experiences these symptoms after eating or at night. The sensation of a heartbeat could be related to the proximity of the stomach to the heart, making it feel like the heartbeat is in the stomach.

Peptic Ulcer Disease: The patient's symptoms could suggest a peptic ulcer, which would cause pain and discomfort in the upper abdomen, nausea, and vomiting. The "knot" sensation might be indicative of an ulcer.
Gastritis: Inflammation of the stomach lining could lead to nausea, vomiting, and abdominal pain, fitting the patient's symptoms.
Functional Dyspepsia: This condition involves recurring signs of indigestion that have no obvious cause, which could align with the patient's symptoms of nausea, vomiting, and upper stomach discomfort.

Pulmonary Embolism: Although less likely, a pulmonary embolism could present with sudden onset of nausea and vomiting, and the sensation of a heartbeat in the upper stomach might be confused with the chest pain associated with a pulmonary embolism. This is a critical diagnosis not to miss due to its high mortality rate.
Myocardial Infarction: Though rare in a 19-year-old, myocardial infarction (heart attack) can present atypically, especially in women, with symptoms such as nausea, vomiting, and a sensation that could be mistaken for a stomach issue.
Ectopic Pregnancy: In a female of childbearing age, an ectopic pregnancy must be considered, especially if there's a possibility of pregnancy. Symptoms can include nausea, vomiting, and abdominal pain.

Gastric Cancer: Extremely rare in a 19-year-old but could present with nausea, vomiting, and abdominal discomfort.
Zollinger-Ellison Syndrome: A rare disorder characterized by excessive gastric acid production leading to severe ulcers, which could fit the patient's symptoms.

Complete Blood Count (CBC) to rule out infection or inflammation
Electrolyte panel to assess for imbalances due to vomiting
Pregnancy test to rule out ectopic pregnancy
Upper GI series or endoscopy to visualize the stomach and esophagus for ulcers or other abnormalities
ECG to evaluate heart function
Imaging studies (CT scan or ultrasound) if suspecting a pulmonary embolism or other serious conditions
Gastrin levels for Zollinger-Ellison Syndrome if other tests are inconclusive and symptoms persist.