Strange Bumps On Egg Yoke

[PoultryPower]

Our Sussex has been laying eggs with these strange white bumps on the yoke. The bumps are white, brown and pink. They look like little round balls all grouped together in a cluster. She shares food and water with other chickens and their eggs don't have this. At first, we thought that a bit of shell had fallen into the yoke because it's kind of the color of her egg shells. It's stuck onto the yoke and present in almost all her eggs.
I tried to take a picture of it best I could. I hope you can see it well enough.
What is it?