Most generally when a person passes out with little to no warning, it suggests a cardiac dysrhythmia, or irregular heart beat. Not everyone who has these is aware of them. They may not feel them. Your Doc is doing the right test, he might recommend a cardioevent monitor, which is like the 24 hour cardiac monitor just worn over a longer period of time. Many times a treadmill stress test to see how your lood Pressure and heart rate respond to the physical stress. If it happens again, you may need even further testing that might include a "Tilt table test" and evaluation by an specialized cardiologist with a focus on checking the electrical pathways of your heart. You have done a great job with exercise and weight loss, but that doesn't totally discount the possibility of disease. You are wise to get this checked out. It is frustrating how often this happens and the actual cause remains unknown.
Tom