I carefully read your thoughtful post.  First of all, I am very sorry that you and Beemer had this terrible event at "obedience" class.  Apparently the border collie's owner needs to take obedience/concentration lessons before  attempting to teach her furkid.   
Not only did Beemer suffer a tramatic event, you did as well.  I have a feeling that you are reminding Beemer, through body language, about the attack.  A dog's first line of communication is through body language and they "read" us like a book.  Be sure that you are completely relaxed and are not "reliving" the attack OR that you even think that Beemer might remembering the attack when you go into the building.  We humans are not able to hide or emotions from out dogs -- I genuinely believe that they sense our anxiety through out actions and even scents that we produce.        |