What Causes Vaginal Bleeding/spotting During Sex After Having An Abortion?

i HAD AN ABORTION A YEAR AGO AND EVER SINCE THEN I'VE BEEN BLEEDING DURING AND SPOTTING AFTER SEX. I HAVE CHECK FOR STDS AND EVERYTHING IS NORMAL I'VE QUESTION THE DOCTOR THAT PERFORM THE ABORTION ABOUT THIS AND HE GAVE ME SOME MEDICINE BUT IT DID NOT DO ANYTHING, WHAT IS WRONG..