Put ice on it (not directly) and that should halt any swelling. The bruise should go away on its on in a week or so.
arteries carry red blood (oxygen), veins carry blue blood (no oxygen), i don't think that blood carries CO2,
Yes, bruises are common after blood withdrawal, depending on the amount.
Irregularly shaped blood cells can cause problems with clotting and proper blood flow. Think of the sickle cell disease. Although you're asking about irregularly sized and not irregularly shaped...
blood is created in the bone marrow and the blood cells then move to the heart to be pumped round the body
It could be the sciatic nerve... go see a chiropractor if it becomes bothersome or pain start shooting into you leg.