I'd also recommend you go to the doctor same as the anon did, OP. This sounds similar to a Cardiogenic Syncope attack thing I had last year after having a blood test - sudden, dramatic drop in blood pressure, feeling dizzy, falling to the ground, apparently going a washed-out grey colour, appearing to have a seizure (which scared the hell out of my mum, who was taking the blood, because I actually have epilepsy o.O).

The doctor said I was fine after checking me out, but she was at least able to explain what happened and now I know more about it in case I have to have another blood test in future (well, I do, since I never technically completed that one, but :S). If you're able to, it might be best you go.

Good luck.