On pelvic examination, I had a retroplaced uterus and my doctor said I had endometriosis. How common is this?

A retroplaced uterus. Is not uncommon, like being left handed. It is possible that on exam your physician felt nodules on the ligaments that support your uterus that could be endometriotic implants. A history of painful periods that resolved while on birth control pills may have also led your doctor to believe this. A retroplaced uterus doesn't = endometriosis though some with endo might have a retoplaced uterus.
Retroflexed uterus. 16-20%of all women have a retroflexed uterus, congenital. Endometriosis, pelvic inflammatory disease among other conditions can creat scaring in the pelvis creating a retroflexion of the uterus.