Why I get pain swollen vein on my left side of my head. It hurts as you touch. I massage it it hurts too much?

An artery. Sounds more like an artery than a vein...Suggest consultation with a doctor...Inflamed arteries in this are are more common than veins and may be a sign of other medical problems.
Arteritis . May not be a vein. Please see physician a group of serious condition associated with arterial pain especially temporal location.