If you lie down then stand up fast and become dizzy, that's from low blood pressure. If you have access to a blood pressure monitor, below 100/65 could cause it. That would go away fairly quickly once your heart catches up.
If it lasts longer, extremely low blood sugar could be the cause. If drinking a glass of orange juice makes it go away, that's a possibility.
If it happens when you're excited or stressed, hyperventilation could cause it.
Pay attention to what you've been doing before it happens. That could give your doc some good clues. If you should faint, you need to go back to the doctor right away.