In cases such as this, there is the possibility of either ectopic ovarian tissue or ovarian tissue left behind during the surgery.
1) During the "heat" have your veterinarian perform vaginal cytology to confirm estrus.
2) Have a board certified radiologist peform an abdominal ultrasound looking for the ovary or ovarian tissue.
3) Have a board certified soft tissue surgeon perform a full abdominal exploratory procedure.
4) Have your veterinarian perform a sex steroid hormone panel.