There are enzymes contained within muscle cells that can be released into the blood in response to vigorous exercise. These enzymes and other break-down products escape from damaged (leaky) muscle cells. It happens in a lot of healthy athletes. The soreness that you feel the day after hard exercise is a sign of damaged muscle (both micro- and macroscopic). Your muscles respond by healing and getting stronger, so that it is harder to injure them next time. The common muscle enzymes released from injury and found in the blood are CPK (creatine phosphokinase) and AST (aspartate aminotransferase).
AST is also found in the liver and can be released in high amounts in diseases causing destruction of liver cells (eg., hepatitis). Blood banks are cautious and do not want to use blood that may contain hepatitis viruses. They perform a number of tests on the blood to look for signs of liver damage (AST, ALT, and sometimes GGT) in addition to specific antibody and antigen testing to look for signs of hepatitis (and HIV) viruses. Blood is rejected if it fails to pass all of the tests. The first time this happens you should consult a physician -- just like the rejection letter says. This is to have your individual case reviewed and to look for any signs of disease.
Most of these enzymes are cleared from the blood within 2 - 3 days in a properly hydrated individual. So avoiding vigorous exercise for several days prior to donating should help you pass the test. In general there are no physiological benefits to donating blood, but your concern for your fellow man is commendable. Blood banks are always in need of blood and athletes tend to have very healthy red blood cells that are resistant to damage and can carry a lot of oxygen. Your blood volume is usually replaced within 2 days after donating, but the red blood cell mass takes longer to re-accumulate -- about 7 - 14 days depending on the individual. After donating it is important to have an adequate diet and a good source of iron because of the metabolic demands of synthesizing new red blood cells. This is particularly important for women and vegetarians. It is probably best not to exercise within 24 hours of donating. Instead, concentrate on fluid replacement. and nutrition. After that, exercise can usually be resumed but at a lower level. Take it easy. Listen to your body and terminate workouts if you feel light-headed or short of breath. Gradually increase back to your routine over the next 1 - 2 weeks. Athletes with health problems or taking medication should consult their doctor regarding specifics about exercise and donating blood.