Ovarian cancer is rare. A woman's lifetime risk is less then 2% (1.3% per U.S. SEER / cancer statistics). Benign (non-cancerous) ovarian cysts are actually more common in post-menopausal women than most people (even doctors) realize. Most cysts resolve on their own in a few months or so. This website may be helpful to you - http://ovaryresearch.com/ovarian_cysts.htm.
Let us know what you find out. Hopefully, it's nothing of concern.