Herpes Zoster (Shingles): This is the most likely diagnosis given the description of pain and a rash with erythematous papules localized to one area of the forehead. Herpes Zoster typically presents with a painful, burning sensation followed by the development of a rash in a dermatomal distribution, which could easily be confined to one side of the forehead.

Contact Dermatitis: This could be a possible diagnosis if the patient has come into contact with an allergen or irritant that has caused a localized reaction on the forehead. The symptoms of contact dermatatitis can include erythematous papules, itching, and burning, which overlap with the patient's symptoms.
Impetigo: A bacterial infection of the skin that can cause erythematous papules and pustules, often accompanied by itching or burning sensations. It's contagious and can occur anywhere on the body, including the face.
Eczema (Atopic Dermatitis): While less likely given the acute onset, eczema can cause erythematous papules and a burning sensation. It often has a chronic course but can have acute flare-ups.

Lupus: Systemic lupus erythematosus (SLE) can cause a malar rash (butterfly-shaped rash across the cheeks and nose) but can also present with lesions on other parts of the face, including the forehead. It's crucial to consider lupus due to its potential for serious systemic complications.
Cellulitis: An infection of the skin and subcutaneous tissues that can cause redness, swelling, and warmth, often accompanied by systemic symptoms like fever. It requires prompt antibiotic treatment to prevent serious complications.
Erythema Migrans (Lyme Disease): Although typically associated with a distinctive "bull's-eye" rash, early stages of erythema migrans can present with a solid erythematous patch, which might be considered in the differential, especially in endemic areas.

Sarcoidosis: A condition that can cause skin lesions, including erythematous papules, among other systemic symptoms. It's rare and would be considered if other, more common diagnoses are ruled out and there are additional systemic symptoms suggestive of sarcoidosis.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. The rash can include erythematous papules and can be localized to the face, including the forehead, but this would be an unusual presentation without other symptoms like muscle weakness.