What can cause a sudden eye blood vessel to pop? never had this happen and now there is a red vessel on the side of my right eye.

Lots of things: In young people, it's generally sneezing, coughing or minor trauma (like getting punched or whacking your head on something). In older people, blood thinners can contribute to this as well. It looks bad but it is generally harmless and will clear up in a week or two.

Often Spontaneous: Small capillaries on the white of the eye may at times burst or leak, often on one eye but sometimes both. These will generally resorb without harm in days to weeks. Often there is no known cause. Sometimes, factors like local trauma (such as a fingernail) or high blood pressure or a bleeding/clotting condition (such as anemia or Aspirin use) or diabetes may be associated. Best to see your eye md.

Often Spontaneous: Such blood vessels on the white of the eye may sometimes leak or burst spontaneously and are called subconjuctival hemorrhages. Often no cause is found and resolution will follow within a few weeks. Best to avoid any rubbing or forces on the eyes. Best to check with your primary doctor or eye md for other risks like diabetes, high blood pressure, anemia, or blood thinner medication effects.

Eye hemorrhage: An isolated sudden "popped" blood vessel on the white part of the eye, as long as it is not painful or associated with vision changes, is not usually a cause for concern. They usually clear within 2-3 weeks. It is reasonable to have this checked out by an eye doctor. Occasionally these hemorrhages can be related to a bleeding disorder or other problem.