What Is Mass In Region Of Extracted Ovary?

I have had a pelvic ultrasound for renal calculi (which showed up in my left kidney) but what has me concerned is the 3cm mass (6.85ml volume) in the region where my right ovary USED to be (I had an Oophorectomy at age 18 - 33 years ago - I KNOW there is no ovary there - the scan films have ??OV on them but it isnt one - what could this be?