like shes broody?
we need some more info...
do you have roosters?
No hens dont know if they're egg is fertile or not, if they go broody (wanna hatch eggs) they will sit on a non-fertile egg, any egg from any animal, THEY WILL EVEN SIT ON NOTHING! No she wont continue laying until you break her broody, which can be done by taking her off of the nest several times a day
is she sleeping on the egg?