Your first issue is you first thought of anxiety about whether this was another concussion. That presets you into a path of anxiety and stress that can cause all of the symptoms you are having.

You don't say if you experienced a "I got my bell rung" sensation.

I would bet your neck was traumatized and you added to that with your early anxious thoughts.

Whether is was or was not a concussion does not matter. You treat the symptoms, not the diagnosis.

If your neck is stiff, treat your stiff neck.