The clenching (holding tension) of any muscles can affect the blood flow limiting the flow of oxygen and nutrients into those areas. Causing headaches & neck aches most likely..

Clenching will not give a concussion..it could make previous symptoms worse.

I would try to find some ways to address the clenching, like a mouth guard, relaxation techniques, awareness so you stop as soon as you notice it..
There could be emotional tension going on that needs to be looked into, or it has just become a habit to break...