Wow thanks for explaining this to me. I just went in to my OBY/GYN for the same reason today. Missed period and on day 55 of my cycle. Had an ultrasound and he found a 5 cm cyst on my left ovary. I think that it was a follicular cyct. I didnt' understand quite what caused it or when it would resolve but your explaination eased my worrying :) He too put me on oral contraceptives.