Sounds like dehydration for sure, to me... but maybe you felt it was traumatic and dissociated a bit? You have GAD and perhaps you were holding your breath to get through the first time (I recall my first time, but had my employer (who had suckered me into donating lol) right there also donating.)

I have given countless times (over 4 gallons total now) and I must sit and drink a few of their drinks and have a cookie or cracker or a few before being well enough to drive etc. Just the way it is?

Good for you to donate!