Thanks for using Healthcaremagic,
It is good that you donated blood, for taking blood from vein a large bore needle is used hence after completion of donation process a firm pressure is required for sufficient time to close the vein puncture.
It is possible some blood has leaked from the site and deposited in subcutaneous space and inflammatory reaction has developed to cause pain called Thrombophlabitis.
This not a serious issue you can apply ice fomentation to get relief and if there is coloration around it there are gels available of heparin
can be used locally to expedite recovery.
You will recover with this problem in few days do not worry at all.
Hope this helps.
Dr. Akhilesh Dubey M.D.