Check the integrity of the cervical ligaments (CAREFULLY and THOROUGHLY) especially the alar ligament.
I have had this happen to myself and it went undiagnosed for 4 years (headaches+++). Chin retractions and upper cervical and deep neck flexor strengthening was the key strategy for me. Also avoidance of end-range lateral cervical flexion (even if you can go there comfortably because without the protection of the alar ligament you can flex too far and it causes irritibility of structures).