If you are lean and regularly do endurance exercise, it's a known phenomenon. It's not of any concern since it won't go to the point of passing out, it's just a curious insight into the control loop that regulates our blood pressure and heartrate and that it's possible to leave the stable area, though not to the point of divergence :)
I do a lot of climbing, am fairly lean, healthy, and have a low resting heart-rate. I've also always had the problem of nearing "blackout" occasionally if I stand up very quickly from zero physical activity - I've never actually passed out, but you have this weird sensation where your vision, balance, and in extreme cases even hearing, fades to various degrees until the pressure stabilises.
I wondered if it would be a similar effect to what happens when a pilot pulls a high-g manoeuvrer.
I have this too; mostly, you just have to train yourself to stay seated for a couple seconds after laying down.
My doctor said the way you die is peeing. You wake up at night, have to pee, make it to the bathroom before getting dizzy, keel over, and hit your head.
