Contact Dermatitis: This is a common condition that occurs when the skin comes into contact with an irritant or allergen, leading to erythema (redness), itching, and sometimes blistering. Given the unilateral presentation, it's plausible that the erythema is a reaction to something that only came into contact with one side of the face.

Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can cause erythema, swelling, warmth, and pain. It often affects one area of the body and can be unilateral on the face.
Erythema Migrans (Lyme Disease): Although typically associated with a distinctive "bull's-eye" rash, early stages can present as unilateral erythema. It's essential to consider this, especially in individuals who spend time outdoors in areas where Lyme disease is prevalent.
Rosacea: A chronic skin condition characterized by facial redness, swelling, and small, pus-filled bumps. It can start on one side of the face and may be triggered by various factors, including sun exposure, stress, and certain foods.

Lupus: Systemic lupus erythematosus (SLE) can cause a butterfly-shaped rash across the cheeks, which might initially appear as unilateral erythema. Missing this diagnosis could lead to delayed treatment of a potentially severe autoimmune disease.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. The rash can be unilateral and may involve the face, making it a critical diagnosis not to miss due to its association with underlying malignancy in some cases.

Erythema Multiforme: A skin condition characterized by target lesions, which can appear as erythema. It's often associated with infections or medications and can have a unilateral presentation, although this is less common.
Sarcoidosis: A disease that can affect multiple parts of the body, including the skin, and may cause erythema or rashes. Facial involvement can be unilateral, but this is relatively rare compared to other manifestations of the disease.