Emotional Flush: This is often the most common reason for sudden redness of the ears, occurring in response to emotional states such as embarrassment, anger, or excitement. It's a physiological response and typically harmless.

Allergic Reaction: An allergic reaction to something like food, drugs, or environmental factors can cause sudden redness and possibly itching or swelling of the ears.
Infection: Infections such as otitis externa (swimmer's ear) or cellulitis can cause redness, swelling, and sometimes pain in the ears.
Irritation or Trauma: Physical irritation from earrings, headphones, or minor injuries can lead to redness and inflammation.

Malignant Otitis Externa: Although rare, this condition can be life-threatening, especially in diabetic or immunocompromised patients. It presents with severe pain and redness of the ear and surrounding tissue.
Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet, and sometimes the ears. It can be primary or secondary to other conditions like myeloproliferative disorders.
Relapsing Polychondritis: An autoimmune disease that can cause inflammation and redness of the cartilaginous parts of the ears, among other symptoms.

Red Ear Syndrome: A rare condition characterized by episodes of redness and burning of the ear, often accompanied by pain. The cause is not well understood.
Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): A rare skin condition that can cause fever, leukocytosis, and tender, erythematous skin lesions, which might include the ears.
Polyarteritis Nodosa: A systemic vasculitis that can affect various parts of the body, including the skin, and might cause redness and other symptoms in the ears due to inflammation of blood vessels.