Brief Answer:
Possible spinal changes.

Detailed Answer:
I read your question carefully and I understand your concern.

Looking at those symptoms the most probable cause are degenerative changes in your cervical spine. Some degree of change in the spine is common with age, it can vary from one person to the other according to genetic factors, the physical load over the years or history of traumatic injury.

These degenerative changes in the spine can cause dizziness and nausea through two possible mechanisms which are compression of the nerve roots leaving the cervical cord, or compression of the vertebral arteries, the vessels which supply with blood the posterior part of your brain and are in close contact with the spine. The last mechanism might also be influenced by narrowing of these vertebral vessels by atherosclerotic plaques.

I recommend you have an x-ray of the cervical spine and (especially if you have vascular risk factors like high blood pressure, diabetes, smoking, high cholesterol) also an ultrasound of the neck vessels.
If degenerative arthritic changes of the spine are confirmed, physical therapy is recommended.

Another possible cause might be issues related to inner ear, like benign positional paroxysmal vertigo, short several seconds bouts of vertigo when head position changes suddenly, but that looks less compatible with your description.

I remain at your disposal for further questions.

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