Almost Blacked Out While Urinating

This has happened to me twice in the last 5 years and it's very specific. I'll go to urinate and while standing there I get dizzy, there's a buzzing in my ears, my heart races and I feel like i'm going to pass out. I hold onto the wall, my vision narrows (or tunnels I guess) and it persists for about 10 seconds or so. Both times it got to the point where I thought I would collapse, but was surprised when it passed and I hadn't.

My background is that i'm a male, 35, good health, 6'2", 200. I've participated in heavy exercise, both aerobic and weight preiodically during the past 5 years - for 2 - 6 months at a time each time, 2-4 times per week - and had no issues with dizziness. I can get up from the couch and run 2 miles without issue, for instance. I do smoke. My blood pressure is normal to below normal. I remember as a teenager a Dr. Put my blood pressure as higher than he read it on a physical, because he said no one would believe it was that low, but no other dr's have said anything. I haven't taken a physical since the army, though 15 years ago.

Is this indicitive of anything. I would think it's an oxygen thing from smoking and then getting up to go to the john, but it hasn't happened any other time, and in both instances I had been up for 3 or more minutes walking around before I went to the rest room.