I'd shop around the images to another specialist and find out what they think the mass could have been that was there one day but gone the next.
Btw, I'm not getting why the push for having an RE do this surgery. While there are REs that are great surgeons, at the end what you need is a great gynecology surgeon. That may or may not be an OB or an RE.
I have a GYN friend whom I sometimes forget and refer to as an OB and she has always politely reminded she's not an OB. She's a GYN surgeon.