Why Is The PAP Smear Abnormal Despite A Negative HPV Test?

I'm 34, 5'3", 166 lbs. I had my first d&c abortion in january 2011. in april 2011, i received my first abnormal pap smear. in addition, my hpv test came back negative. since the abortion, i have had more pain related to ovulation and menstruation. i have engaged in risky sexual behavior. what might be causing an abnormal pap smear if the hpv test came back negative?