Hen walking backwards

I have a Light Sussex who takes a step forward, then goes into reverse for five or six steps; then forward a step or two and backwards, repeating this until she bumps her rear against something, when she trots forward for a while.

I thought at first it was snow blindness, but she is still doing it long after the snow has cleared.  She did improve, but this morning is worse again. 

She is eating and drinking, but I think she's losing weight.

Has anyone any experience of this?

Many thanks, cathangirl

Thank you 8doubles.  Liskeard (LS) is 18 months old and only started doing this when we had the snow.  Feel more reassured now.
cathangirl