Can you get a picture? It could be the yolk sac. Does it look like this?
This is an unabsorbed yolk sac. (I found the picture here on BYC, by the way :) ) And here's another thread on unabsorbed yolk sacs. https://www.backyardchickens.com/t/469499/help-with-an-unabsorbed-yolk-sac
ETA-...