Gastroenteritis: This is a common cause of nausea and vomiting in the elderly, and the low platelet count could be secondary to a viral infection or sepsis. The normal ultrasound and other laboratory findings do not rule out this diagnosis.

Medication-induced nausea and vomiting: Many medications can cause nausea and vomiting, especially in the elderly. The low platelet count could be due to a medication side effect, such as heparin-induced thrombocytopenia.
Gastritis or peptic ulcer disease: These conditions are common in the elderly and can cause nausea and vomiting. The low platelet count could be secondary to bleeding or inflammation.
Infection: Infections such as pneumonia or urinary tract infections can cause nausea and vomiting in the elderly, and the low platelet count could be secondary to sepsis.

Hemorrhagic stroke or cerebral vasculitis: Although the ultrasound is normal, a hemorrhagic stroke or cerebral vasculitis could cause nausea and vomiting, and the low platelet count could be secondary to a consumptive coagulopathy.
Sepsis: Sepsis can cause nausea and vomiting, and the low platelet count could be secondary to disseminated intravascular coagulation (DIC).
Acute abdomen: Conditions such as appendicitis or bowel obstruction can cause nausea and vomiting, and the low platelet count could be secondary to sepsis or inflammation.

Thrombotic thrombocytopenic purpura (TTP): This is a rare condition characterized by thrombocytopenia, microangiopathic hemolytic anemia, and renal failure. Nausea and vomiting could be symptoms of TTP.
Heparin-induced thrombocytopenia (HIT): This is a rare condition that occurs in patients who have been exposed to heparin. The low platelet count could be secondary to HIT, and nausea and vomiting could be symptoms of the underlying condition.
Vasculitis: Conditions such as giant cell arteritis or Wegener's granulomatosis can cause nausea and vomiting, and the low platelet count could be secondary to inflammation or sepsis.