A small lump in the vein on the top of my arm very sore

It appeared over night a lump that seems to be in the vein on the top of my forearm if i touch it or apply pressure it doesn't go down but it moves. It is very sore n if i peg out washing or anything like that i get a sharp pain. It now looks almost bruised i have had it for 10 days has me rather worried it is even hurting now as i type it is about 2cm long but not very high. Thanx