13 year old on birth control?

The risks for heart attack, strokes and blood clots are, as far as I am aware, mostly only concerning to women roughly double your age and older. Since you were not born with any congenital heart problems as a result of your premature birth it seems unlikely that you would be at any kind of risk for those type of health problems and your doctor should have considered that possibility when they wrote the prescription. If you trust your doctor then it's safe to assume your doctor did indeed consider that possibility.