When I had mine done it was scheduled for 9 something and I had fasted because I assumed that is what they have you do for most blood tests. My anxiety level was through the roof to so that probably didn't help. I have social anxiety and just sitting there in the waiting room with that big crowd of people was stressng me out. They only took three vials but it was immediatly after they started taking the first that I started to feel that way. I had never had issues before.

I think it had to do with a number of things

One, anxiety.
Two small needle.
Three, they tied that rubber arm band around my arm SO tight that my fingers were starting to go numb from squeezing that ball they give you before they even started to prick me. So maybe It was from the tightness and the numb feeling and having the oxygen cut off in my arm or something.