I was thinking the egg could be fertilized but I gather the eggs daily and refrigerate them so know there isn't a chick inside. But could this be what the spot is on the egg? I will post a picture when I have one with the spot.
I have one girl that lays an egg that has an unusual spot on the yoke. The egg looks normal but the yoik has a little dark brown speck between the yoke and the little white cord attached to the yoke. (not actually on the yoke) I would imagine it is a fertilized egg because I do have a rooster...