I can understand your concern. The best way to find if this scar tissue or related to some infection/disease is a visual/physical examination.
Which I am sure your doctor has conducted already. As you have tested negative for all sexually transmitted diseases (STDs) and a pap smear
has also returned a normal result, I do not think there should be anything to worry about. But for your satisfaction you could go about one last investigation; a biopsy
. This would confirm/rue out any abnormality if present.
Hope this helps.