[Turning back from his conversation with the third party, he accidentally elbowed me in the back of the head]

Use Logic & reasoning to convince yourself - an accidental brushing bump from an elbow is not a concussive force.
Even if the barber said ouch , he may have hit his funny bone and that zings quite a bit..
A purposed elbow hit will hurt, but probably not concussive unless a trained attacker does it..