I'd go back to your doctor and talk to them again about the ongoing problem. It is possible that it is stress related. Like with Amie635, hers sounds like it's from anxiety, as she'd have a panic attack. Though really there are SO many possible causes of dizziness. certain medications can cause it, dehydration, high or low blood pressure, hypoglycemia, anemia, etc. I get dizzy a lot myself, but have made some lifestyle changes that have helped a little. good luck.
I have the same problem and have done since I was about 6 years old. It's difficult to explain but I get the dizzy feeling you explain at really random times, suddenly and it startles me to the point it that sometimes it ends in me having a panic attack. I've been to hospital numerous times and they couldn't diagnose me, all they said was it is stress related and taught me some breathing exersises.