[mscat]

My 3 yr old yorkie dislikes men who are strangers. She barks loud and is very angry when a male stranger even comes close to where we live. She has not bitten anybody , but that is a real possibility , considering how protective she is. I agree that their is something wrong here. Your yorkie is very perceptive. Either your dog had a very bad experience that has left her traumatised with the BF, or she senses something about him that is very wrong. I understand that your BF and dog are very important to you, however, I would go with what my dog is trying to tell me before anyone else.
Your BF could crouch down to your dogs level and be very still. See if she goes towards him or not. I strongly doubt it. I think the damage is already done. And something did happen. Even if it were minor , the trust has been lost. Maybe BF walked too fast , or raised his voice , something really scared your dog.
If your dog has not ever behaved this way before with anyone then their is problem with BF.