Lump like a 5 pi coin on dogs side

[pinkyponkyplink]

What could this be? I’ve booked the vets for Tuesday but I’m worried. It is like a 5 p in the skin. It moves when I lift the skin up and it’s kind of inside the skin layer. No colour but feels under the skin.

My greyhound had this when she was a puppy and it turned out to be a cyst.

Started off like a small coin but then went sort of like a cherry.

I should say she's almost 9 now and never had another one after having it removed.