Its very tricky to figure out isn't it?
I did a VO2Max test whilst hooked up to an ECG on a treadmill on Tuesday and then had an episode of vasovagal syncope after where my heart rate went down to 36bpm for 10 mins or so.
Before that i only had a rise of about 15bpm on standing.
I have felt a little rough since then so out myself on bed rest. Over the last 3 days i have gradually felt better and it correlates pretty well to the amount of orthostatic rise
As you say, is this just an indication of some other process in the body, who knows.