I have had my Yorkshire Terrier for nearly 2 years and these last 2 weeks I've had a problem with him. When taking him for walks I have come across other dog walkers who we have seen many times before and my dog has played with. We the last 2 Weeks these same dogs have been out during our walks and my Yorkshire terrier has growled and gone for these dogs. It has never happened before and I am really concerned by his behaviour because they are not new dogs.. He has seen them before and been fine in the past.