If you hit the head any any sort of an angle , and sometimes angle doesn't matter...
Have you ever read about upper cervical adjustments , atlas/axis, c1 c2 adjustment?

You may have misaligned those top vertebra during the bump..
It can cause many odd symptoms as well as the body changing over time to re-balance the weight of the head..

more info here-
http://neurotalk.psychcentral.com/post735715-11.html