I have a bulging vein on the top left side of my head been there for 3 months and doctor told me it's stress.i'm really concerned as it kinda hurts?

Vein or artery: Are you sure its a vein? If so, this may indicate nothing except that you are well hydrated. It could also mean higher than normal pressures in your chest, but this is unlikely. Given your age, its most likely just developmental. Follow up with your doctor if you remain concerned. If its an artery, it can be due to inflammation and your doctor may suggest some blood work.