You may be describing a sign of neck instability. The top two vertebrae in the spine make up a unit called the atlantoaxial joint. One vertebra is slipping forward of the other one. This occurs with weakening of the capsules and ligaments around the facet joints.
Trauma and/or deterioration can occur on one side or both. The weight of the head during neck flexion moves the bones into the subluxed position. Extending the head and neck when you look up causes a clunk as the bones shift back to their normal places.
Many patients with this sign also report stiffness and "crunching" or crepitus in the neck during neck movements. There may be neck pain and even sleep apnea (breath stops) from pressure on the spinal cord and traction on the brainstem.
Until you know for sure what's causing this problem, it may be best to avoid neck flexion. Call your doctor today and report this symptom. Get a follow-up appointment for a re-evaluation. Pay attention to any other symptoms that might be present. Report these to the doctor no matter how small or insignificant you may think they are.