Anxiety or Panic Disorder: The sudden onset of feeling extremely cold and shivering without an apparent environmental cause, relieved by gabapentin (which is sometimes used off-label for anxiety), suggests a possible anxiety or panic disorder. These conditions can manifest with physical symptoms that mimic other medical issues.

Hypothyroidism: Although less directly linked to gabapentin relief, hypothyroidism can cause feelings of cold intolerance. It's a common condition that could explain the symptoms if other signs such as weight gain, fatigue, or dry skin are present.
Fibromyalgia: This chronic condition is characterized by widespread musculoskeletal pain, fatigue, and sensitivity to temperature. Gabapentin is often used to treat fibromyalgia, which could explain the relief experienced by the patient.
Neuropathic Pain: Conditions that cause neuropathic pain, such as diabetic neuropathy or postherpetic neuralgia, can sometimes present with unusual sensations including feeling cold. Gabapentin is a first-line treatment for neuropathic pain, which supports this diagnosis.

Hypoglycemia: Although typically associated with diabetes, hypoglycemia can cause shivering, sweating, and feeling cold among other symptoms. It's crucial to rule out hypoglycemia because it can be life-threatening if not promptly treated.
Adrenal Insufficiency: This is a rare condition where the adrenal glands do not produce enough cortisol and, in some cases, aldosterone. It can lead to a variety of symptoms including feeling cold, and it's critical to diagnose and treat to prevent adrenal crisis.
Spinal Cord Injury or Lesion: Any injury or lesion affecting the spinal cord can disrupt normal bodily functions, including temperature regulation. This would be a critical diagnosis to consider, especially if there's a history of trauma or if other neurological symptoms are present.

Multiple Sclerosis: This autoimmune disease can affect any part of the brain and spinal cord, leading to a wide range of potential symptoms, including abnormal sensations such as feeling cold.
Mitochondrial Myopathies: These are a group of disorders caused by mitochondrial dysfunction, affecting muscle and nerve cells. Some types can lead to abnormal temperature regulation and could potentially cause the symptoms described.
Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet, but in some cases, it might present with feelings of cold, especially if the condition is not well-managed or is in a different stage of its cycle.