Is there a word for the instant after you are slapped and are deciding how to react?

Solution 1:

I think the most appropriate word would be "Stunned".

Astonish or shock (someone) so that they are temporarily unable to react.

Solution 2:

I think the phrase taken aback can be used this situation.

TheFreeDictionary defines this phrase as:

confused or surprised by something unexpected

It offers the following examples:

When Mary told me the news, I was taken aback for a moment. When I told my parents I was married, they were completely taken aback. Company executives have been taken aback by the criticism. I asked him directly if he was looking for someone with my skills, and I think he was kind of taken aback.

There are also examples of this phrase on Google used to describe people who were slapped.