It is probably just membrane, and no shell. You can use a clean finger and lube it to insert a finger into her vent to feel for any remnants, but it has probably already come out.
I tried Scratch and Peck layer feed once. There were seeds that they would eat, but the powdery parts (probably vitamins and minerals and other nutritional additives,) they just left. So, I prefer a more homogenous feed such as crumbles or pellets, where each bite has all of the parts they...
Usually when this happens, the other chickens see it and pull it out. It should come out in her droppings. It sounds like she is either not getting enough calcium in her diet or her shell gland is not working properly. When they have shell-less eggs, they can break inside if they are having...
It is probably an egg membrane from an egg that collapsed inside. I would try to pull it out, but don’t force it. Do you see any egg yolk or white in the coop or on her vent? Is she coming out of or going into a molt? I would be tempted to give her a human calcium tablet with vitamin D or Tums...