Your neck muscles are not strong enough to cause an injury unless you were to shake your head back and forth violently for an extended period of time. Those momentary jerks in response to a startle are easily tolerated. Any symptoms are more likely due to a startle anxiety reaction where you brain kicks into fight or flight mode and is overwhelmed with stress hormones.

So, no continuous shaking your head to rock music or head shaking to check your heads tolerance.

Your anxiety about this is like making a startle response last for days. Try to relax and find something productive to think about.