Thank you so much for your speedy responses, you lovely pet lovers you.

Zara had a worming tablet from the vet a month ago; and the vet saw no evidence of worms the other day, and she took a sample for analysis and found no infection. Though to be honest it sounds like worms to me, and not something more serious, after looking at Other Causes of Hindquarter Conditions:

She is going to see the vet again tomorrow, to take 'further tests'. But i wanted to be a bit more informed, so thank you again for all your advice.