Brief Answer:
Many potential causes, inner ear issues most common.

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

The causes of dizziness can be really many. It is important to determine whether it is vertigo (sensation of spinning movement), lack of balance, feeling of blacking out or simply light-headedness. One of the most common causes at your age, especially if not known to have any vascular or heart issues are causes related to disorders of the inner ear. Most common one benign positional paroxysmal vertigo, which has these characteristic spells of vertigo and dizziness usually related to a change in head position. Physical exam including a maneuver called Dix-Hallpike maneuver often evidences that.

Other causes may include problems with your blood vessels, their narrowing from atherosclerotic plaques or from degenerative changes of the spine leading to their compression can also cause such symptoms due to insufficient lack of blood flow in the posterior regions of the brain, putting you at risk for stroke.

Heart arrhythmias also could lead to such dizzy spells with feeling of blacking out. Low blood pressure as well, although you mention a strong pulse which renders that less likely. Also since you had surgery recently I suppose some screening blood tests were done to exclude causes like anemia.

So as you can see the causes can be multiple, often in inner ear related ones benign but some causes deserve serious attention. Differentiation is difficult at time, physical examination by a physician is necessary to check for signs related to inner ear, accompanying neurological deficits, heart irregular rhythm. According to the findings imaging tests might be indicated.

I remain at your disposal for further questions.

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