For years, I've always been plagued with sudden dizziness, most often when getting up from a reclined position or if I've been kneeling (working on the bike, out in the garage tinkering on something, etc.) but since my surgery two years ago I've had my dizzy spells with increasing frequency and intensity to the point that I have to remind myself to get up slowly to allow the blood to drop to my lower extremities more slowly rather than suddenly.
I consider myself to be in excellent health (aside from PCa, of course) with textbook blood pressure, healthy weight (6 feet, 190 pounds), etc. I exercise an hour a day, 5 days a week, and I take no medications other than a pom extract and a daily multi-vitamin. I stopped taking Ambien about three weeks ago now after having been on them for about 5 years straight (5 mg nightly). Yay, me!
I figure it's just a part of growing older.