From a triathlete's non-medical point of view: I had an episode earlier this year when after a run I went "lights out." Scary stuff. I suggest get tests, tests, and then more tests, and lay off the training until you figure this thing out. That was one of the hardest parts for me, pulling back, but being around for my family ranks way above whatever fitness I might lose while trying to get to the bottom of the problem. And whether you have a family or not, triathlon, no matter how much we may love it, is not worth risking your life over. Since you are an RN, sounds like you have lots of access to medical advice, but I still think having at least one doc who is actually "on the clock" working your case is a good idea. Who knows what combination of factors could have led to your episode, but I think you are wise to be cautious, especially about swimming, when these come on with zero warning. I hope you get to the bottom of it soon, and that it is something that can be taken care of.