It looks like an egg could've cracked when it was in the womb. Or maybe she's malnutrientioned? Sometimes that can happen. But you might need to take her to the vet just in case.
I hope I helped you.
I dont know how to give you the link but I posted pixs just the same as yours. Look at the post here. Titled "INTERESTING!What is it an egg?Check it out its Graphic-Pictures"its by me,red rooster farm I just put a reply on my to bring it to the top for you to look up. Anna