For the last 3 days, one of my white Lethorns has been having something unusual with her eggs. On the side of the egg, there's a slightly raised bump. It's about the size of a dime, and it's slightly gray instead of being pure white like the rest of the egg. It's not hugely raised but you can easily see and feel it. One of the eggs like this also had a hole right there - the center of the bump had fallen in. I cracked that egg and it looked totally normal. This hen has been laying since last October. Any idea what this is or what causes it? The girls are on layer pellets, they have oyster shell and they get Avia 2000 Charge vitamins in their water at least once a week.