Normally the vein puncture at the blood donation site does not pain after an hour or two. There maybe slight bluish discoloration which is not painful. People often forget that there was a needle puncture.
However, if there is pain persisting then probably there is infection or inflammation of vein. This is called thrombophlebitis and is a swelling or inflammation of a vein caused by a blood clot. Dopplar ultrasound is quite conclusive in this—otherwise venography is performed for confirmed diagnosis. Often clinicians can diagnose this on their own. Normally analgesics, anti-inflammatory and some anticoagulant (please never take this on your own as they can cause bleeding into organs) are given for this. Avoid hanging your hand down for a long time. Take care!
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