I would have expected your vet to come up with some thoughts and suggestions even if there is nothing physically wrong with her. Have there been any other changes in your household recently that may have upset her? Is there any new tension in the household that she may be feeling and responding to? Is your vet absolutely sure that there is nothing wrong from a medical perspective?
Try and think hard about changes in the home that may be an issue and see if, by making some alterations (if possible), she then reverts. There will be a cause it is just a matter of finding it!
There are some good websites with sound advice from people who really know the breed and the rescue sites http://www.wiccaweys.co.uk/ http://www.bordercollierescue.org/ http://www.fostbc.org.uk/
also try http://www.groveshilldogwhisperers.co.uk/sarahjenkins.html
I only know the people at Wiccaweys by reputation but I believe they are first class ditto BC Rescue. FOSTBC is Barbara Sykes who is a total expert and Sarah Jenkins is superb.
Good luck and keep us posted!