They are English words in an English phrase which we understand and use. It looks correct to me. The idea here is of a "mental fog".

The example you've given, "I was in a fog for several days after my son's accident," is perfectly correct an natural-sounding.

Typically we use these two phrases when a person is confused, for example after suffering concussion, or coming out of surgery, or perhaps they have just woken up. She was in a fog when I met her, I don't think she knew where she was. After the accident, he seemed to be in a haze for several days. I woke them at 3am, and they both seemed to be in a fog, but they startled me when they opened the door holding a gun. That really threw me into a haze.