What Causes Presence Of "endometrial Cells" On Cervical Pap Smear?

History of HPV with cervical dysplasia...had a leap procedure about 6 years ago with only 1 bad pap since procedure that corrected on its on within 6 months. Just had a pap come back with endometrial cells present on cervix and have to have a biopsy later this month. Should I be concerned or is this normal? GYN has never used the term "endometrial cells" before...not sure what to think. 39 years old, 5' 3", 178 lbs.