Most people will feel okay after the donation. Some donors might experience slight giddiness. For all blood donors, the nurse will ask them to rest on the donation bench for 10 mins after their donation. 

If you feel slightly giddy after the donation, continue to rest on the donation bench after the 10 mins or at the refreshment area. Only leave when you feel ok. It is not necessary to eat more after the blood donation to compensate. Blood volume can be replaced just by drinking plenty of fluids. 

You should avoid strenuous exercise after donating. Your athletic performance might be slightly lower for the next 3 days. 

Here are some tips on post-donation care you can read.