I don't think it would be considered a birth defect. If you had a scar on the back of your eye at birth due to an in utero infection it was be a congenital retina scar. If the scar is inactive it should not be painful. I suggest you have your parents set you up with an ophthalmologist for a eye exam and a discussion with you and your parents on what the scar is and if it has any significance also to evaluate the eye pain you're having.
JCHMD