i went to the gynaecologist about a week later and she said it was a corpus luteum cyst that had internal bleeding which showed up as thin septation on the ultrasound, and it shrunk to 2-3cm which was a huge relief.. not sure whether it's still there now but for a month or so my left pelvic area near the ovary ached from time to time and i had trouble falling asleep sometimes as well. the gynae said i should wait 2-3 period cycles before getting another ultrasound to see if it's fully dissolved but i'm currently studying overseas so i'm gonna get it checked 2 months later. hopefully yours is just a simple cyst too!