3 pap smears in the last year and half

I've had my yearly examine last yr it came back abnormal and then had another one in 6 month it came back good had my yearly again 2 weeks ago. And pap came back bad again? Why? What could be the cause of this? I mean when they did another test after the abnormal one called a colposcopy it showed some abnormal cells but nothing to worry about or so said the doctor. But my HPV is negative?