Welcome to Health Care Magic.
Based on the information provided by you, the bruise
could be due to the needle which was used for collecting blood sample. Improper insertion of needle damages the vessel wall and cause swelling and bruise.
Do not worry . The description of bruise suggests it is dissolving. Hot fomentation or ice pack will help in reducing the pain and swelling. It would take atleast 10-12 days for the bruise to disappear. Heparin
will help in dissolving the bruise and swelling but usually not recommended as it may cause rashes in few people.
In case if the bruise and swelling are increasing in size see the doctor immediately and take his advice.
Wish you a speedy recovery.