Anxiety or Panic Disorder: A lightheaded, uneasy feeling, especially if localized to the back of the head, could be indicative of anxiety or a panic disorder. These conditions are common and can manifest with a variety of physical symptoms, including lightheadedness and unease.

Benign Paroxysmal Positional Vertigo (BPPV): Although BPPV typically presents with vertigo, some patients might describe their symptoms as a lightheaded or uneasy feeling, especially if the vertigo is mild or atypical. The symptom's relation to head position is a key diagnostic clue.
Vestibular Migraine: This condition can cause a variety of symptoms including vertigo, lightheadedness, and headache. The uneasy feeling in the back of the head could be a precursor to or part of a vestibular migraine.
Cervicogenic Dizziness: This refers to dizziness that originates from the cervical spine. It can be caused by neck pain or dysfunction and might be perceived as an uneasy feeling in the back of the head, especially if there's associated neck stiffness or pain.

Transient Ischemic Attack (TIA) or Stroke: Although less common, a sudden onset of lightheadedness or an uneasy feeling could be a symptom of a TIA or stroke, particularly if accompanied by other neurological deficits. The posterior circulation (vertebrobasilar system) strokes can present with vertigo or lightheadedness.
Subarachnoid Hemorrhage: A sudden, severe headache (often described as "the worst headache of my life") can be accompanied by an uneasy feeling or lightheadedness. Although the headache is typically more prominent, any sudden severe neurological symptom warrants immediate investigation.
Multiple Sclerosis: This condition can present with a wide range of neurological symptoms, including vertigo, lightheadedness, or an uneasy feeling, especially if there's involvement of the brainstem or cerebellum.

Chiari Malformation: A structural defect in the cerebellum that controls balance can cause symptoms of lightheadedness, vertigo, or an uneasy feeling, particularly with coughing, sneezing, or straining.
Acoustic Neuroma: A benign tumor on the nerve connecting the inner ear to the brain can cause vertigo, lightheadedness, or an uneasy feeling, along with hearing loss and tinnitus.
Vascular Malformations or Arteriovenous Malformations (AVMs): These are abnormal connections between arteries and veins, which can cause a variety of neurological symptoms depending on their location, including lightheadedness or an uneasy feeling if they affect areas responsible for balance or equilibrium.