That depends on the situation... And how the slap was done. Most men (including myself) do not slap well. It is in our physiology to hit things our bones and muscles are heavy and hard when we slap we tend do drive the weight of our hand which causes a much harder and different type of hit than we intend. I have started just using my fingers instead of the palm of my hand because of this.. And I use a light rubber whip or belt for that purpose the point being to bring a wash of high intensity sensation without deep bruising. Kimmi (kimmigirl here) and I have worked out what works for us. But I am always afraid i will spank her too hard or in the wrong way.
As for the shockingly hard slap.. what was the particular situation.. does he have a habit of doing it... what does it do for you or to you. does it scare you in a way that makes you unconfortable... Does he derive pleasure from hurting you in an unhealthy way... You should contemplate the situation and answer these questions.