I think there can be a variety of causes for symptoms.
The startle can cause a muscle reaction that can effect a subtle neck injury.
The chemical flow from the anxiety of the startle response can take a while to correct. One may calm the physical part and thought part of the startle but the chemical (adrenaline) part can take much longer.

I know how long it can take to get the chemical response to settle down. I experience it myself. I do not get anxious about the impact. I only react to the startle. The PCS brain can easily over-react chemically. I've experienced times when I am 'on edge' for a day or longer even though I can not thinking about the bump.

It can be annoying to have to put up with these longer responses but I don't think there is anything that can be done to settle the body without a medication. I have used propranolol to help with the settling if it lasts to long or is too intense.