Cervical Strain or Sprain: The patient's presentation of non-radiating neck pain and headaches could be indicative of a musculoskeletal issue such as a strain or sprain, which are common causes of neck pain. The absence of radiation and a negative Spurling test (which assesses for cervical radiculopathy) supports this diagnosis.

Tension Headache: Given the presence of headaches, tension headaches could be a contributing factor or the primary cause of the patient's symptoms. Tension headaches are often associated with neck pain and can be exacerbated by stress or poor posture.
Cervicogenic Headache: This condition involves headaches that are referred from the cervical spine. The patient's neck pain and headaches could be related to a cervicogenic cause, especially if there is a history of trauma or if the headaches are triggered by specific neck movements.
Stress or Anxiety: Night sweats can be associated with stress or anxiety, which could also contribute to or exacerbate neck pain and headaches.

Meningitis: Although less likely, meningitis is a critical diagnosis that must be considered, especially if the patient presents with severe headaches, fever, and night sweats. A stiff neck (nuchal rigidity) is a classic sign, but early in the course, symptoms might be more subtle.
Lymphoma or Other Malignancies: Night sweats are a classic "B" symptom of lymphoma, along with fever and weight loss. While less common, it's crucial to consider malignancies in the differential diagnosis, especially if other risk factors are present.
Infectious Diseases (e.g., Tuberculosis): Certain infectious diseases can cause neck pain, night sweats, and headaches. Tuberculosis, for example, can present with systemic symptoms including night sweats and can affect the cervical spine.

Cervical Dystonia: A rare condition characterized by involuntary muscle contractions in the neck, leading to abnormal postures and pain. While it could explain the neck pain, it's less likely to account for all the symptoms.
Temporal Arteritis: This condition, also known as giant cell arteritis, can cause headaches and is more common in older adults. However, it would be unusual for it to present with isolated neck pain on one side without other symptoms like jaw claudication or visual disturbances.
Neck or Spinal Infections (e.g., Abscess, Osteomyelitis): These are serious conditions that could present with neck pain, fever, and systemic symptoms like night sweats. They are less common but require prompt diagnosis and treatment to prevent serious complications.