Migraine with aura: This condition often presents with headache, can be accompanied by facial pain, and sometimes a rash if the patient has a condition like migraine-related dermatological manifestations. The pain can radiate to the back of the head.

Tension Headache: While typically not associated with a rash, tension headaches can cause pain that radiates from the face to the back of the head and could be exacerbated by or coincident with a dermatological condition causing a rash.
Sinusitis: Inflammation of the sinuses can cause facial pain and headache, and if the sinuses are infected, it could potentially lead to a rash in severe cases, though this is less common.
Temporal Arteritis: This condition causes headache and facial pain, particularly in older adults, and can be associated with visual disturbances and jaw claudication. A rash is not a primary symptom but could be present in the context of giant cell arteritis with polymyalgia rheumatica.

Meningitis: Although less likely, meningitis is a critical diagnosis that must be considered, especially if the patient has fever, stiff neck, and altered mental status in addition to headache and rash. Facial pain is less common but can occur.
Encephalitis: Similar to meningitis, encephalitis is a serious condition that requires prompt diagnosis and treatment. Symptoms can include headache, fever, confusion, and sometimes a rash.
Lyme Disease: If the patient has been exposed to ticks, Lyme disease could be a consideration, especially if the rash is characteristic of erythema migrans. Headache and facial pain can occur, especially if there is involvement of the cranial nerves.

Erythromelalgia: A rare disorder characterized by burning pain, heat, and redness in the hands and feet, and sometimes the face. It's less likely to cause headache radiating to the back of the head but could be considered in complex cases.
Trigeminal Neuralgia: While primarily causing facial pain, in rare instances, the pain can be so severe that it leads to headache. A rash would be an unusual accompaniment but could be present due to other conditions.
Sjögren's Syndrome: An autoimmune disorder that can cause facial pain due to salivary gland inflammation and potentially headache. A rash could be part of the systemic manifestations, though it's not a primary symptom.