Cavencruiser,

First, your high level of anxiety can cause all of these symptoms. A backhand would have to be extremely hard to cause a concussion and would likely cause seriously bruised knuckles or even broken fingers.

Second, you are correct to not take a smack to the head lightly from a health point. Nobody should be disciplined with a smack to the head, no matter how light. I suggest you establish an 'off limits' contact agreement with your father. The head and neck and spine are OFF LIMITS. A warning of a promise to contact an authority if it happens again would be worth considering. You know your father but you also need to protect yourself.

This smack sounds like it was to the upper neck and cervical spine area. That is a very dangerous area full of arteries and nerves.

You should seek help with your anxiety and overly focused attention to detail, especially health issues. That can make for a miserable life. Learning to let go and accept the ups and downs and focus on the bigger picture when possible will be beneficial to your future.