Don't take this as a diagnosis, only that no matter what you should see a doctor to be checked out. It can be any number of things.
In my case it was sort of related to a drop in blood pressure.
As a kid, sometimes (and only sometimes) after a vigorous workout, or even just getting up quickly, I would what I would describe as "grey out". My vision sort of disappeared - instead all I basically would see was something between static and a grey/brown plaid. It usually only lasted a few seconds, during which I'd also feel light headed and a bit nauseous. After exercising if it occured, it might result in a short episode of vomiting. But it never lasted long, and I'd be perfectly fine. Never really thought anything was "wrong". Of course, my dad would get mad at me because it seemed like I was trying to get out of exercise. My varsity soccer coach would also think the same.
After graduating college, I went to a gym with a buddy to work out. I did a strong workout on an excercise bike that had a pulse monitor. Towards the end it said my pulse jumped to 270. I figured it was wrong - it was just a cheap thing. But when I finished, I got up, walked away, and then had to grab on to a railing to keep from collapsing on the floor. The grey out happened again. After a few minutes it went away, except for the nausea which lasted a lot longer this time.
So I went to see a doctor. My new primary doctor was in fact a cardiologist in general, so he liked to get a "baseline" ECG for his patients. My baseline was anything but normal. From just a 5 second ECG, he could see something unusual. So I went back to do a stress test. Before I even started the stress test, they saw a spontaneous significant increase in heart rate. I was showing signs of tachycardia and was diagnosed with Wolff-Parkinson White Syndrome. I was referred to one of the top cardiologists at the Brigham in Boston, and it was determined that my situation was pretty serious - there was a possibility of spontaneous cardiac arrest. WPW is also something that can be more symptomatic and serious as you get older.
To put it simply, I had a short circuit in my heart. When your heart beats normally, an electrical charge starts at a node at the top of your heart and travels downward via a bundle of fibers. At the top, it causes the atriums of the heart to contract, pushing blood into the ventricles. It generally takes a certain amount of time to get to the lower heart, when it causes the ventricles to contract, pushing the blood out of the heart while the atriums fill again. The two steps give the pumping action.
In my case, there was another pathway the electrical charge could take. It didn't always, but in stressful situations it was more likely. It also passed through faster. In those situations, the ventricles would contract too soon, before the blood had completely passed from the atriums - and effectively not pump the blood efficiently.
Fortunately, I've been treated and I am symptom free for almost 20 years now. The treatment was "experimental" at the time but is now routine.
And it was the same thing that cause Meat Loaf to pass out on stage one time, so he and I have something in common
Wow, I spent way too long typing that out...but like I said, don't treat it as nothing, because it might not be. It's probably nothing serious, but its something to be looked at.