Hi I Discovered A Lump Behind My Belly Button ,

Hi

I discovered a lump behind my belly button , that has moved from above my belly button.
My doctor sent me for a ultrasound and they said they need to let a specialist to look at what they found, so booked in for another scan on the 26th may.
What he did say to me is that there is no mass and not a hernia or a cyst.
It showed on scan to be a white circular ball.
I suffer with polycystic kidney disease, and knew it was not a cyst , but felt like I was dealing with a inexperienced doctor.

Any idea of what it could be please?

I have no pain only discomfort when it's messed with.

I discovered the lump when I had servere pain above my belly button, but it settled after 2 weeks.

Many thanks
Nicole