Not to worry, this happens and it's fairly common among chickens. Sometimes because of an environmental stressor or other external factor, other times seemingly spontaneously. I had an identical egg one morning from a BCM...it was her first egg. She then laid a bigger, normal egg that same day. I originally thought 2 hens had begun laying, but they were both from Mossy! Her "pullet egg" stage was over in a matter of hours and from then on she laid large eggs.
Occasionally I will get little eggs like that from hens. Usually it doesn't continue, and it's more of a isolated occasion.