I have been a regular blood donor since being 18, and have now donated sixty times or so (I have lost count) and can remember that very first time. I was not overly anxious about it; coming from a medical family, I knew there was nothing scary. Just after giving the blood I felt a bit light headed and silly - apt to giggle a bit. After ten minutes with the tea and biscuits I felt normal again. They say you should not drive or operate dangerous machines for a bit after, because of this giddiness.
I have never noticed any lessening of mental acuity after giving blood. I must emphasise, that giddy feeling was most noticeable the first time, I've never had it like that since, and it only lasted a short time. I accidently had a whiff of amyl nitrite (poppers) from a friend once, and that was ten times worse, complete with hours of thumping headaches afterwards!