I've only had one bad reaction after donating, and not since (and that was immediately after when I passed out- now I put ice packs on my neck during donation and drink a gatorade).
I donate every 8 weeks- I usually work out before I dontate, then again the next day. I've never experienced what you did 2 days after. I think your reaction may be a result of your own body's reaction to the depletion of the blood (and maybe it's just mad at you for taking a pint away from it ). Are you low body weight, or have normally low blood pressure? Torrilin could be right on.
Anywho- hope it doesn't happen again. Keep on donating, tho!

Oh- and we went to ancient ruins in Mexico and couldn't donate for a year. I was totally bummed. If I'd known that- I wouldn't have gone (to the ruins- I still would've gone to Mexico). I understand that they just want to be extra cautious tho- if I'd developed malaria months after returning I sure wouldn't want to have given that to someone.
I didn't mind the inconvenience- I'm glad they have so many precautions- makes it safer in the end.