These dark marks seen on my 1 1/2 year old Isa Brown’s egg shells are new. They also seem a bit more fragile than normal. She is eating Prince organic layer crumbles, free ranges when there’s no snow, and gets sunflower seeds and organic greens a few times a week as a treat. These have been showing up almost daily for last couple weeks. She is acting fine and none of my other hens are laying like this. Should I be concerned?