Sunburn: The most common cause of erythema (redness) on the face and ear, especially if the individual has been exposed to the sun without adequate protection. The left side could be more exposed depending on the individual's activities or the side of the face that is more prone to sun exposure.

Contact Dermatitis: An allergic reaction or irritation from substances like soaps, cosmetics, or plants that come into contact with the skin, causing redness and inflammation.
Rosacea: A chronic skin condition characterized by facial redness, often affecting the cheeks, nose, chin, and forehead, but can also involve the ears.
Eczema (Atopic Dermatitis): A condition that makes your skin red and itchy, and can appear anywhere on the body, including the face and ears.
Insect Bites or Stings: Can cause localized redness and swelling, especially if there's an allergic reaction to the bite or sting.

Lymphoma (Cutaneous T-Cell Lymphoma): Although rare, certain types of lymphoma can present with skin manifestations, including erythema. Early diagnosis is crucial for effective treatment.
Lupus: Systemic lupus erythematosus (SLE) can cause a butterfly-shaped rash across the cheeks and nose, but it can also affect other areas, including the ears. Missing this diagnosis could lead to significant morbidity.
Cellulitis: A bacterial skin infection that can cause redness, swelling, and warmth, typically on one side of the face or body part. It requires prompt antibiotic treatment to prevent serious complications.

Erythema Multiforme: An immune-mediated disorder that can cause target-like lesions on the skin, often triggered by infections or medications. It's rare but can be serious.
Sjögren's Syndrome: An autoimmune disorder that can cause skin manifestations, including erythema, among other symptoms like dry eyes and mouth.
Polychondritis (Relapsing Polychondritis): A rare disorder that causes inflammation of cartilage and can affect the ears, nose, and other parts of the body, leading to redness and swelling.