It could be due to internal 'scarring" of some sort after the accident, it takes time for that to appear and cause troubles.

I don't know how you feel about chiropractic, but if nothing is an issue in the c spine then soft tissue could be the cause.

Might be some trigger points, muscle spasms , misalignments..
an expert chiropractor can be helpful or an advanced PT.

I had the best relief from very good chiro & advanced PT people.