A very small number of donors, less than half of one percent, experience slight discomfort during or immediately after donating. The required questions and tests lessen the chance of a bad effect from giving or receiving blood. For this reason, it is extremely important that prospective donors answer all questions accurately and thoroughly. Yes, you will receive a card by mail within a week of donation that explains your blood type. I play sports.
Athletes can donate as regularly as anyone else.
However, you should refrain from giving blood on the day you are scheduled to compete. Instead, donate after practice and drink plenty of water to avoid dehydration. I may faint. The likelihood of this happening is minimal. Eat before donating, and drink extra fluids after the donation.