You are over analyzing everything.

Defining acute phase and recovery phase is over analyzing it.
The only time the experts suggest extra care is the first 24 to 48 hours. After that, they suggest getting back to life as symptoms allow.

Walking briskly is not a risky action. You may have sparked an adrenaline rush (a part of the anxiety response). If you have an upper neck injury, the walking could cause in increase in inflammation. If you were almost running, that is usually a less impacting action.

You have convinced yourself that you acted stupidly. This increased frustration and anxiety. You have attributed symptoms to actions that were likely meaningless.

Your anxiety has an answer to reject every reason you should not be anxious. Your ex GF bolting up did not say she launched into the headboard. When somebody bolts up in bed, they move away from the headboard. I have occasional night terrors and never bolted up toward the headboard. See how you manipulate the issue to make it sound risky and support your anxiety?

Go ahead. Keep convincing yourself that your life is over because you rushed to catch a bus. Or, you slept odd. Or the ceiling fell down and landed on your head. Or a zombie came into your room and beat you in the head.

Yes, you may need to take some minor precautions such as protecting your head and neck during bumpy rides. I have done this for 15 years. Not a big deal. When I get caught off guard, I shake it off and go on with life.

Go on with yours.