I adopted my 8 year old male pit bull last November. He looks scary but has always been great with people & kids. We're still working on other dogs. Anyways, we have a routine in the am that includes a long walk, affection & food. On Mondays, however, I also take him to my office in the am while the housekeeper is at my house. He has gotten really good about not barking at people outside, but he has recently started growling at people entering my small office space. He has never shown any aggression to people before & is really scaring everyone including me! What can I do?