Thanks for writing to us.
It seems during drawing of blood, a traumatic injury
to blood vessel might have occured that led to extravasation of blood from vessels and pooling of blood in adjacent tissues leading or bruising and haematoma
formation. Ideally in few weeks time it should heal on own but to expedite the process, do warm water compresses over the area and apply thrombophob cream. Also take vitamin c supplement
for few days.
Hope this helped.