Carcinoid Syndrome: This condition is characterized by flushing, diarrhea, wheezing, and heart valve abnormalities, often associated with a neuroendocrine tumor. The flushing can be triggered by certain foods, stress, or other factors, making it a plausible explanation for postprandial facial flushing, especially if it occurs after lunch and is associated with other symptoms like diarrhea.

Gustatory Flushing: This is a common, benign condition where certain foods or drinks trigger flushing due to the release of vasodilatory substances. It's more likely to occur after consuming specific types of food or alcohol.
Menopausal Hot Flashes: Although these are more common in women and not strictly postprandial, they can be triggered by certain foods or drinks and could potentially be mistaken for postprandial flushing if they occur after lunch.
Medication-Induced Flushing: Certain medications, such as niacin, can cause flushing as a side effect. If the patient has recently started a new medication, this could be a likely cause.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic flushing, hypertension, and other symptoms. Although rare, missing this diagnosis could be catastrophic due to the potential for severe hypertension and cardiac complications.
Medullary Thyroid Carcinoma: This rare cancer can produce calcitonin, which may cause flushing among other symptoms. Early detection is crucial for treatment and prognosis.
Mastocytosis: A condition characterized by an accumulation of mast cells in one or more organs, which can cause flushing, among other symptoms, due to the release of histamine and other mediators.

Harlequin Syndrome: A rare condition characterized by unilateral flushing and sweating, typically affecting one side of the face. It's often associated with a disruption in the sympathetic nervous system.
Erythromelalgia: A disorder that causes burning pain, heat, and redness in the hands and feet, and sometimes the face, which could be mistaken for flushing. It's more common in the extremities but can involve the face in some cases.
Autonomic Dysfunction: Various conditions affecting the autonomic nervous system can lead to abnormal blood flow regulation, potentially causing flushing. These conditions are diverse and can include diabetic neuropathy, multiple system atrophy, and others.