Viral Gastroenteritis: This condition, often referred to as the stomach flu, is characterized by inflammation of the stomach and intestines, usually caused by a viral infection. Symptoms include headache, nausea, vomiting, queasy stomach, and chills, making it the most likely diagnosis given the combination of symptoms.

Influenza: The flu can cause a wide range of symptoms including headache, nausea, queasy stomach, and chills. It's a common condition that fits the described symptoms.
Food Poisoning: This condition occurs after eating contaminated food, leading to symptoms like headache, queasy stomach, nausea, vomiting, and chills. The onset of symptoms can be rapid, depending on the type of contaminant.
Migraine: While primarily known for causing severe headaches, migraines can also be accompanied by nausea, queasiness, and sometimes chills, especially if the migraine triggers a systemic response.

Meningitis: Although less common, meningitis is a potentially life-threatening condition that requires immediate medical attention. Early symptoms can include headache, nausea, vomiting, and chills. It's crucial to consider this diagnosis to prevent severe outcomes.
Appendicitis: While the primary symptom of appendicitis is abdominal pain, it can sometimes present with nausea, vomiting, and chills. If the appendix ruptures, it can lead to severe complications, making it a critical diagnosis not to miss.
Sepsis: Sepsis is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. Early symptoms can be non-specific, including headache, queasy stomach, and chills, making prompt recognition crucial.

Brain Tumor or Cyst: In rare cases, a brain tumor or cyst can cause symptoms like headache, nausea, and queasiness due to increased intracranial pressure or local irritation of brain structures.
Temporal Arteritis: This condition involves inflammation of the blood vessels in the head, leading to severe headaches, and can sometimes be accompanied by systemic symptoms like fever and chills.
Encephalitis: An inflammation of the brain tissue, usually caused by viral infections, can present with headache, fever, chills, and sometimes nausea and vomiting. It's a rare but serious condition requiring prompt medical intervention.