Soft shell

One of my girls looked very fluffed up and miserable just now just before they are all due to go to bed. She then layed a pale soft shelled egg which I got rid of straight away. This is 12 hours before her normal laying time. Last night there was a fox next door making a LOT of horrible shrieking noises. Could this be why she laid the egg, is it like delayed shock? She is normally fine, they all have oyster shell in their diet and free range quite a bit in the garden. Can anyone help please?

Thanks for your reassurance Sarah. Buttercup did another softie overnight it was under her place on the perch. But this morning she laid a normal one.