Sorry. The vaso-vagal reflex causes a sudden drop in blood pressure due to pain and/or stress. Not generally associated with any life or health threatening diseases, unless you fall off a mountain or get sucked into a treadmill because of it.
But if in doubt, check it out with a doc in real life.
Ok, so I just read about “vasal-vagal syncope” which is a medical way of saying that I succumbed to the physiological mechanism of fainting, which means that my blood pressure, and heartrate were too slow for the activity I was performing.
Would the onset of a cold produce this effect?
Ok, so maybe my bruised knee was causing enough pain as I was working it out to stimulate this reflex, or maybe the cold, or maybe I just had a Tony Soprano type psychological episode where my daughter playing with the stuffed animal reminded me of youthful plush toy induced trauma, but no big deal unless it becomes common, right?
No biggie unless there are recurrences or other symptoms, like more light-headedness, sweats, palpitations, nausea, or visual changes. Again, I can’t diagnose anything over a message board, and would urge you to consult a doc if you have further symptoms or concerns. But pain, stress, and infection can explain the symptoms you describe here. I just can’t verify that they are indeed the true explanation. I’d guess that given the info presented, they’re the most likely diagnosis. But I’ve guessed wrong before too.
So now you’ve got more info to make whatever choice you want to make.
Good luck. And at least try to not break your neck, ok? Recalling your welcome of me here nearly 3 years ago, I’ve grown strangely fond of your presence on this message board.
Another reason to use an excercise bike instead. 
And I positively cringed reading about your poor knee there, Scylla.
When I was a kid, I banged the funny bone in my arm quite hard (fooling around in Sunday School, as a matter of fact) and I went down like a sack of potatoes. Maybe you managed to re-create that effect with your injured knee on the treadmill, Scylla. I don’t know if we have a similar thing in the knee as the elbow, but I know I’ve greyed out from banging my knee in just the right spot in the past.