tiny egg, no yolk?

Hi all,  one of my hens layed an egg the size of a cherry tomato, very strange, i cracked it open to find just the white, and a tiny speck of yellow, she has been laying normal size eggs up till now, has anyone got any ideas on this??? thanks. 

if you take a look part way down this page you will see a photo of one that I got a couple of years ago.

http://chat.allotment-garden.org/index.php?topic=47692.15