Where did THIS tiny egg come from???

When the hens get old this happens time to time. Usually there is no yolk. I've had a few of them time to time. It's normal.

 

It might be "what is called a 'peewee' (see link below). This will be a normal egg just miniature in size. It occurs occasionally (usually with young pullets just entering lay) however, if the eggs contents are not that of a 'normal' egg (consisting of a yolk and white) yet to all outward appearances looking like a 'normal' egg with a shell, then you have what is often called a "rooster egg". The contents are not that of a 'normal' egg and this has usually been caused by a bit of sloughed off material (often from an inflamed or irritated oviduct) having found its way into the reproductive tract, which will in turn stimulate the oviduct to produce albumen /'white' (see also below re: 'rubber' eggs) and form an eggshell encompassing it. This 'rooster'egg' will usually consist only of eggwhite, though on occasion you may see what appears to be 'yolk' at the core, around which the white and shell have formed. When there is no shell around it and it looks like a rubbery lump of gunk (but the hen has clucked over it as if she has "laid"a proper egg) then this occurs for various reason but usually the bird has become an "internal layer" often as a result of damage to the reproductive tract from Infectious Bronchitis. As long as it does not occur regularly then it is usually nothing to worry about (unless the bird is also not laying regular in combination with this).
http://www.aeb.org/LearnMore/EggFacts.htm

http://www.georgiaeggs.org/pages/eggcyclo.html
the EGG encyclopedia!!!