Thanks for the additional information. It is helpful.
There are some medical conditions that can cause sudden aggression and those may be a factor. Unfortunately, these would not be able to be ruled out without testing. Now I don't believe these are the cause but I have to mention them.
It sounds like your dog may be having issues with dominance aggression. It does sound like fear may be playing a part as well. Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you. In this case, your boyfriend. If your boyfriend doesn't live there, she may even feel like she is protecting you.
If you really want to stop the behavior, there has to be changes. First off, she can not be on the furniture. Dogs that are allowed on furniture (even if put on the furniture) tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack. So the first thing is to not allow he rhigher that the humans or even on the same level. Attach a leash and use it to remove her from the furniture. Give a correction in the form of a quick tug and firm "NO" when she attempts to get on and a treat when she starts not trying to get on the furniture. Thus you are providing negative reinforcement for the getting on the furniture and positive reinforcement for the desired behavior (not attempting to get on the furniture).
There are other ways to regain the dominant position in the house as well. The best way is to start obedience training. While a formal training class is great, you can start obedience training without a formal class. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Now if your boyfriend is willing, he should do the training of her. Have him use hot dog slices for treats rather than store bought ones. Obedience training helps her gain self confidence and once she learns what he expects when he gives a command she should be less fearful as well. In addition, if he can pitch his voice a little higher when talking to her, she should be less fearful. Lower tones of voice are reprimands such as growls where higher pitched tones like yips are invitations to play and fun tones. Also try and not lean over the dog. Dominance is often shown by one dog leaning over the other dog's neck. When a human leans over a dog, they may interpret that as a dominant gesture as well.
If he will take over her care including feeding and affection, she'll quickly start treating him as her owner and start to allow him to do anything he wants.
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.