Gastroenteritis: Although no fever or apparent infection is mentioned, viral gastroenteritis can sometimes present without fever, especially in adults. The extreme nausea and fatigue could be due to dehydration and electrolyte imbalances resulting from persistent vomiting and decreased oral intake.

Food Poisoning: Similar to gastroenteritis, food poisoning can cause severe nausea and fatigue. It often results from consuming contaminated food and can lead to dehydration if not managed properly.
Viral Syndrome: A non-specific viral illness can cause fatigue and nausea without the typical signs of infection like fever. These syndromes are common and can resolve on their own with rest and hydration.
Medication Side Effects: Certain medications can cause nausea and fatigue as side effects. If the patient has recently started a new medication, this could be a likely cause.
Anxiety or Stress: Severe anxiety or stress can manifest physically as nausea and fatigue. This is particularly relevant if there have been significant life changes or stressors recently.

Appendicitis: Although classically presenting with abdominal pain, nausea, and fever, some cases of appendicitis can present atypically, especially in the early stages or in certain populations like the elderly or pregnant women. It's crucial to consider this diagnosis to avoid missing a potential surgical emergency.
Pregnancy: In women of childbearing age, pregnancy should always be considered, especially if there's a possibility of conception. Morning sickness can sometimes be severe and prolonged.
Diabetic Ketoacidosis (DKA) or Hyperosmolar Hyperglycemic State (HHS): These conditions can present with nausea and fatigue, among other symptoms, and are medical emergencies requiring prompt treatment.
Myocardial Infarction or Cardiac Issues: While less common, cardiac conditions can sometimes present with atypical symptoms such as nausea and fatigue, especially in women or diabetic patients.

Adrenal Insufficiency: A condition where the adrenal glands do not produce adequate amounts of hormones, leading to a variety of symptoms including nausea, fatigue, and potentially life-threatening complications if not treated.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, which can present with nausea, fatigue, and other systemic symptoms.
Cyclic Vomiting Syndrome: A rare condition characterized by recurrent episodes of severe nausea and vomiting, often with no apparent cause.
Neurological Conditions: Certain neurological conditions, such as migraines or vestibular disorders, can cause nausea and fatigue, though these would typically be accompanied by other neurological symptoms.