Your hen is laying Wind Eggs.
A wind egg is the colloquial name for a fully formed egg that does not have a yolk in it. These are generally much smaller than normal eggs and are a blip in egg production, either at the beginning or end of lay when the hormone level has not stabilised. A young hen will settle down and begin to lay proper eggs once the days really start to lengthen. Known by different names such as “dwarf eggs”, “wind eggs”, and “fart eggs”, these tiny and often yolk-less eggs are generally resultant of a new layer with an immature reproductive system -or- {in an older hen} the product of a situation where a piece of reproductive tract has been sloughed off and then treated like an egg, thus prompting it to be encased within albumin, membranes, and shell {giving it the exact appearance of an egg, just much smaller and generally without yolk}.
They are often the result of stress, immunity malfunctions, rising illness, or vitamin deficiencies in adult hens. Though Oyster Shell isn't directly the root of the problem, wind eggs can still be linked to a lack of calcium. Do you supply your hens with calcium (oyster shell) on a regular basis? Is there any way your hen could be abnormally stressed? How is she functioning otherwise?
~Sarah