Thanks for the replies - that's exactly what I would have thought and was ready to give her Metacam tonight, however .....
Vet popped in on his way home for a look and guess what? ....... flippin maggots in the tail !!!
Can't believe that could cause her to move strangely like that - I had a look in the wool on either side last night and didn't see anything and she firmly held her tail to the side.
Quick snip with the scissors and put on some Spot On and vet said that should be that.