Know this is a long time ago you asked but I had the same symptoms and turns out I did actually pass out a couple of times, while not exercising.  After 2nd event in a few months they checked me in overnight and found out I had Sick Sinus Syndrome and my HR was cratering, to the point it would actually stop for several seconds at a time, most often when I was sleeping (so much fun waking up with a crash team in your room).  Anyway had a pacemaker put in and no problems since.  BTW, I was a 50 year old, active, otherwise healthy male who cycles (just did a total of about 100 miles this weekend, 60 flat on Sat and 35 climbing on Sunday), hikes, plays basketball and such.    So if you have something like this talk to your doc.  They might schedule for you to wear a halter monitor or schedule you for something called a tilt table test.  Both are definitely worth your while.