A positional headache red flag is a headache that worsens when you stand up and subsides when you lie down, or vice versa.
A headache that worsens when you stand up can indicate secondary conditions, such as intracranial hypotension.
A headache that worsens when you lie down can be a sign of increased ICP, such as from idiopathic intracranial hypertension (IIH) or a brain tumor.