A lump appeared on my left wrist.it seems like a vein which is very swollen and sort of round.it pains when pressed and my arm is a little numb?

Painful lump: I suggest you see your family doctor. There a re a number of possibilities, including both benign and malignant tumors. See your doctor, or a local clinic where you live, or an emergency room doctor who can evaluate it and refer you to an appropriate doctor.

Ganglion: What you are describing sounds like a ganglion rather than a vein. Ganglions are cysts that form in joints usually at the wrist. See a durgeion or hand surgeon for an evaluation.