While it is not possible to answer your question exactly, the description as a circle suggests a Weiss ring. This is the ligament that used to go around the optic nerve when the vitreous was attached through it. Googe Weiss Ring and also 'images' to see what they look like
I have had victrectomy in each eye for retina detachment, followed by 2 weeks of positioning on my side for two weeks (both eyes separate occurrences). To me, and merely a guess, you may be experiencing the effect of a gas bubble that was injected and will get smaller until it’s gone. Mine lasted 3 months. After that I got cataract almost immediately.
I wouldn’t hesitate contacting your surgeon. They expect calls like this. I think I had mine on speed dial :). My questions were always answered and if there was concern they had me come in.