Hello, Thanks for your query. I would appreciate your efforts for taking initiative to donate blood and to be of great service to others! Sometimes it is difficult to locate vein especially if it lies deep or the nurse is less experienced .That leads to leakage of blood from vein and hence haematoma formation.inflammation of vein or nerve irritation may lead to pain but they are self limiting and usually subside on their own! It will be okay in 5 -7 days apply topical antibiotic cream twice a day DONOT worry!take care! I do hope that you have found something helpful and I will be glad to answer any further query. Take care