Might want to get checked out for orthostatic (postural) hypotension. I was sitting in a hot shower once, stood up and passed out cold. Something as simple as a B-12 deficiency can cause it. Combined with low BP and heart rate in endurance athletes it can be more pronounced. In my case it just runs in the family. But it's manageable if I watch what I'm doing.. like not standing up too quickly after sitting for a bit, avoiding the hot tub/sauna, etc.