Postmenopausal Cysts-can they be cancerous?

I'm 45 years old, and had hysterectomy at 29 due to both ovaries having numerous cysts which were causing all kinds of problems, and they wouldn't go away.  I had my tubes tied earlier since I never wanted children so I was fine having the surgery at a young age.  A small piece of the left ovary was able to be salvaged, and was left in as I didn't want hormone issues back then at such a young age.  On the small piece of ovary, is a 4 cm cysts that appears to be fluid-filled.  It was discovered this past July, but at that time was 3 cm.  In a one month timeframe it grew to 4 cm.  My doctor is very conservative, and just keeps doing sonograms every 3 months. Blood work showed that I hit menopause a couple of years ago.  A few weeks ago, the cyst started throbbing a bit, but now, in the past two weeks, I'm getting a really deep sickening pain that comes and goes, and at times, I feel tremendous pressure in that area,and I get nauseated.

I'm at the point of looking for another doctor, but have one more appointment with her next week to see what she says this time.  How dangerous is this problem in a post menopausal woman?
What is the hold up in removing this thing?  I backed her into a corner last time, and she admitted she could not guarantee it wasn't cancer, but the sonograms show it to be fluid-filled, so the chances would be low.

Sorry this is so long-I'm very frustrated, and am wondering if I'm crazy or something for feeling like my health is being put in jeopardy.  Thanks