Re: Vagal reaction?
If this is the first time it has happened to you, I would think it was because of the stress of the unknown test. That combined with the fact you have not eaten or taken any fluids.
During your next test, try looking away from the area where the blood is being taken. Also drinking a normal amount of water would help keep you hydrated.
If these do not help and you get dizzy again, let someone know if you have time. That warm feeling just before you pass out is all of the blood going out into the tissue of the body. When that happens, not enough blood is getting to the brain and you pass out. As soon as you get that blood flowing back to your brain you should wake back up.
Pilots when they pull high amounts of Gs (force of gravity) can pass out. They can practice certain maneuvers to force blood into the upper part of their body and wear protective equipment (anti G suits) which squeeze the lower part of the body and thus not letting as much blood pool in the legs. I know this from first hand experience while flying in the back seat of an F-4 fighter while I was in the Air Force. While I was flying the aircraft, the front seater (pilot) told me we had to put “More” Gs on the jet in order to level out at our assigned altitude. We were doing what is called a Split S maneuver where you turn the jet upside down and pull back on the stick to come out of the bottom part of a loop. I had only been pulling around 4.5 Gs (4 and 1/2 times the normal force of gravity) and we needed a little bit more in order to hit our assigned altitude. As I pulled harder on the stick (hydraulic assisted) the G-meter went up to 6.5 Gs. At that point the pilot said that was enough and we both where trying to breath as the air was being pushed out of our lungs. I began to “Gray Out” which is when the brain starts to starve for blood. Your vision becomes as if you were looking down two paper towel tubes. If we would have continued on, we could have both passed out. If we had not had our G-suits on and were trying to do our lower body squeeze maneuvers, we could have blacked out even before we got to 4 Gs. (4 X normal gravity)
Speaking of, you can try tightening the muscles in your legs and stomach when you get that warm flush feeling. This might help fight off the syncope/fainting. If this does not help, you will awaken once blood flow has returned to the brain.
In some people, including me, a sudden sharp pain can cause us to black out. This has happened during medical procedures and once while I was bending over at the waist. A small sharp pain in my back put me on the floor in less than 3 seconds and I awoke when my cat came over to see what was going on. In my case, all of my blood vessels dilate at once and not enough blood can be sent to the brain. The docs think that my heart arrhythmia (too slow of heart beat) plus my low blood pressure contributes to my syncope/fainting. This was further examined during a tilt-table test when I passed out and my heart stopped beating for 15 seconds before they lowered the table to the horizontal position. The heart started back on its own once the blood started flowing back to the brain.
If your problems still persist after you have taken the above measures, you might need to have further tests to see if there might be any underlining problems. I would bet though it was just because you had not eaten or drank fluids (water) and the stress of an unfamiliar test.
Good luck with the rest of your blood tests.