Hm, Are you sure you just never noticed it? Have you lost any weight or anything? Typically a bulging vein on the face is no cause for concern. On the temple or head, they can be a sign of stress or pressure. They are actually fairly common! I guess the main thing I'd be concerned with is if your blood pressure is rising. Normally you can't see anything like that but some with that issue have a protruding blood vessel where you describe. Probably not the best time to head out to get your bp checked but next time you can, check on it. But remember it is very common for many people and is typically not worrisome. https://www.healthline.com/health/forehead-vein#causes
Hello~I would check with your GP, especially since they just occurred.