ovarian cyst-could it be cancerous

I have been diagnosed with a big ovarian cyst just on Friday. Thursday night I had extreme pain in my left side. The doctor thought it was a kidney stone due to the pain I was in, and had me on morphine and toradol.  Now I already have had a hysterctomy and I only have my left ovary.  The reason the left it in was, I get blood clots and can't be on hormone replacement therepy.They sent me home with vicodon and a nausea pill so I can at least try to eat and told me to rest and call my doc on Monday. My question is, since I don't ovalate to have my period, why did I get a cyst, and is it possible that it is cancerous due to the conditions I mentioned above? Thanks capt29