In my experience, this sounds like a neck issue. The upper neck can become unstable from the whiplash. It takes quite a long time to heal and will not likely heal properly unless you avoid poor head and neck postures, especially during rest and sleep when the neck muscles are relaxed. Some gentle neck therapy can be helpful. An upper cervical chiropractor or physical therapist / physiatrist who understand subtle neck injuries can be helpful. The treatment should be gentle. A PT with myofacial release skills may help as you may have spasming muscles that make it difficult for the cervical vertebra to settle into proper position.

I don't think this can even close to being a concussion.

It does not sound like this startled you. Startle response is an anxiety response. It does not sound like you had an anxiety reaction.

For some, a muscle relaxing med (Soma/carisoprodol or the like) can help if the muscles are tense. They are not to be used for long term. With a muscle relaxing med, you have to be very disciplined with rest and sleep posture.

If this is a neck issue, it can take a long time to heal, especially if you can not find someone to help you understand how to take care of your neck.

My best to you.