Hen lying tilted to one side.........

Tonight, one of my girls was lying under the ladder, but tilted to one side with a leg straight out.  I checked her over and put her into the house with the other nine hens, but she just lay there in a similar position.  Earlier she had been squabbling over a muddy "dust" hole.  
I and my OH can find no injury.  Obviously we will keep an eye on her tomorrow.
Is it usual for hens to rest like this often?  (I know they do when dusting.
Thank you in advance of your answers.
Cathangirl

The symptoms you describe are A typical as described in the book for Mareks disease. Theres masses of information on here with regards ti this but I'm afraid its terminal.