Dehydration: This is the most likely diagnosis because vomiting can lead to significant loss of fluids and electrolytes, causing dehydration. Dehydration can lead to a decrease in blood volume, which in turn can cause a decrease in blood pressure, resulting in lightheadedness.

Vasovagal Syncope: Vomiting can trigger a vasovagal response, leading to a sudden drop in heart rate and blood pressure, causing lightheadedness or fainting.
Electrolyte Imbalance: Vomiting can lead to an imbalance of essential electrolytes such as potassium, sodium, and chloride, which can affect blood pressure and cause lightheadedness.
Gastroenteritis: Infections that cause vomiting can also lead to lightheadedness due to dehydration and electrolyte imbalances.

Myocardial Infarction or Cardiac Arrhythmia: Although less common, vomiting can be a symptom of a heart attack or arrhythmia, especially in older adults. Lightheadedness in this context could be due to decreased cardiac output.
Bleeding or Hemorrhage: Vomiting blood (hematemesis) could indicate a serious underlying condition such as a bleeding ulcer. Lightheadedness could result from significant blood loss.
Intracranial Hemorrhage: Rarely, vomiting and lightheadedness could be symptoms of a brain hemorrhage, especially if accompanied by severe headache or neurological deficits.

Adrenal Insufficiency: This condition, where the adrenal glands do not produce enough cortisol, can lead to vomiting, dehydration, and lightheadedness, especially during times of stress.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of vomiting, hypertension, and lightheadedness due to the excessive release of catecholamines.
Autonomic Dysfunction: Conditions affecting the autonomic nervous system, such as dysautonomia, can lead to orthostatic intolerance, causing lightheadedness after vomiting due to an inappropriate response to changes in position.