Quite normal to happen in hours and be gone in days.
It's usually a result of poorly regulated blood pressure in someone his age but could happen from your late 30's for same reason
Have you ever gotten shampoo in your eyes? This can cause a quite severe bursting of blood vessels in the eye, which usually disappears within hours.
If you got punched or had some other trauma directed at your eye when you where 16? Did you actually have more serious damage than a burst blood vessel? Like detatched retina? This kind of damage is much slower to heal.