Thanks for writing in.
Well accidentally hitting a branch of vein or artery
can happen. You should consider yourself fortunate that giving a drug meant for intramuscular use into vein or artery did not cause other problems. Hematoma
is a minor complication, use of ice packs would help it subside. The half dose needs to be given intramuscular- request a colleague who is experienced to supervise you.
If hematoma does not reduce in size in 2 days consult a doctor. One has to consider then that it might be infected or it might be something different which was pre existent and was not noticed before injection.
It was good of you to notice it even before completed the injection. Do not worry, it is benign situation and you have no reason to feel guilty about it.
With Best Wishes
Dr Anil Grover