Gastroenteritis: This is the most likely diagnosis due to the combination of nausea, vomiting, and diarrhea, which are classic symptoms of gastroenteritis. The low sodium level (hyponatremia) can be attributed to the loss of sodium through vomiting and diarrhea.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, nausea, vomiting, and electrolyte imbalances, including hyponatremia.
Medication-induced Side Effects: Certain medications, such as diuretics, can cause hyponatremia and gastrointestinal symptoms.
Adrenal Insufficiency: This condition can lead to hyponatremia and gastrointestinal symptoms like nausea and vomiting, especially during periods of stress or infection.

SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion): Although less common, SIADH can cause severe hyponatremia and is often associated with nausea and vomiting. It's crucial to diagnose and treat promptly to avoid severe complications.
Addisonian Crisis: A life-threatening condition that requires immediate attention, characterized by severe adrenal insufficiency, which can present with hyponatremia, nausea, vomiting, and diarrhea.
Intestinal Obstruction: Can cause severe vomiting and diarrhea, leading to electrolyte imbalances, including hyponatremia. It's a surgical emergency.

Congenital Adrenal Hyperplasia: A rare genetic disorder that can cause electrolyte imbalances, including hyponatremia, and gastrointestinal symptoms.
Cystic Fibrosis: Although primarily known for respiratory symptoms, cystic fibrosis can also cause gastrointestinal symptoms, including diarrhea and malabsorption, potentially leading to hyponatremia.
VIPoma (Vasoactive Intestinal Peptide Tumor): A rare pancreatic tumor that secretes vasoactive intestinal peptide, causing severe diarrhea, hypokalemia, and achlorhydria, which can lead to hyponatremia.