Thanks for writing in.
The condition can be due to extravasation of blood in the skin while inserting cannula for blood transplant.It will gradually resolve with time.The pain however should not occur .This pain can be due to thrombophlebitis
You should try simple analgesic
like ibuprofen with paracetamol
for a day or two.
If problem persists then you must visit a dermatologist
or physician for proper diagnosis and treatment.