Hope you're felling well today, Sarge. I believe Mr. Giese is dead on about staying on the doctors for a better answer.

A friend of mine had a similar experience recently. He is active and athletic as I assume you are if you bike regularly. His doctor wasn't able to pinpoint a cause either, but suggested he was dehydrated and may have had a flu virus. Yeah, a flu virus. No other symptoms - just passed out. He's fine now and back out working and playing. I hope it is as simple for you!