It's probably a vein on the side of the forehead that became bulky, because the arteries are deeper and are not visible as the more superficial veins, which lie just below the skin.
This enlargement of the vein may be caused by stress, exercise, heat, or it can be just a characteristic of the person.
Leaner individuals have more "skipped" and visible veins, but this has to do with low amount of adipose tissue (fat) below the skin, which leaves the veins more exposed.
Usually muscular people also have veins more visible, but since there is very little muscle on the forehead, this should not be the case in this situation.
A query with a angiologist or vascular surgeon can clarify whether this increase in the volume of the vein is something natural or pathological.