Can you get venous air embolism from giving blood if they inserted some air in at the beginning? would you be able to feel it? how long would it take?

Air embolism: Unfortunately, injecting significant amounts of air into the venous system will produce an "immediate" effect with an air embolism which can produce death.

Unlikely: To get a true air embolism you are talking about a big mass of air, not the tiny bubble that might be present in a needle inserted when you give blood. When/if it ever happened it would be felt in minutes if not immediately.