When you say that your blood pressure is fine, have you experimented and measured it just as you stand up? After my mechanical valve was fitted to replace a stenotic Aortic valve 3 years ago, my previously slightly high blood pressure dropped to be borderline low, I found that I would feel dizzy just after standing up. Soon after, I joined a clinical trial for a cholesterol lowering drug, and one advantage of doing so is the more frequent comprehensive blood testing that they do during the trial, and they tested my pressure while sitting and then standing. They found a small drop, but as the pressure was already low it dropped sufficiently to cause the dizziness. The advice was (a) to add some salt to my diet, to increase blood pressure, and (b) to stand still upon standing for a few seconds, to allow blood pressure to recover a normal level.
Apparently the important reading is the lower one, the Diastolic reading, with results below 60 likely to lead to dizziness.
If there is a significant difference between the standing and sitting readings I recall it can be medically significant and you should tell your doctor. As an aside, I was on a blood pressure lowering tablet, which they reduced to a small daily dose but wanted me to continue to take it despite having low pressure. This was because it "provides other benefits".
