My chickens have done the same thing every couple of months they'll lay an odd one and then it will go back to normal laying. Its most likely fine just make sure you are giving them plenty of food, treats, and some type of calcium like crushed up ouster shells and believe it or not you can actually feed them there own shells with the yoke or without it. Whenever your cooking instead of throwing the shells away, give them to your chickens. They are using alot of their calcium to make their eggs. If you crush it up and mix it in their feed it will help them with their eggs a lot so its less common that will happen. I wish you luck!