Hi thanks for your question.
The condition which you have debeloped during blood donation
is called as thrombophilibitis that is inflammation of vein.It has also invoved the surrounding structures.
Give rest to your arm,hot foamentation and local application of heprinized ointment like thromophob is recommended.
Hopefully it will get settle dawn in one week time.In case of pain,you can take mild pain killer like paracetamol
Hope this answers your question.