Bella seems to be what we refer to a dominant little dogs. Owners often don't put the same rules on smaller dogs as they would larger ones like Buster. However, 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.
It sounds like your dog may be having issues with dominance aggression. 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 or others 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 your dog's case, she isn't allowing others near you as she considers you HER possession.
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 her higher that the humans or even on the same level. In addition, humans shouldn't be on the floor with her either. A small short stool is enough to keep them higher than the dog when petting the dog. 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).
This is temporary as once she realizes that it is your decision on when she can be on the furniture and not hers you can then invite her on the furniture AFTER she performs a command and earns the right to be on the couch or bed, etc. However, she has to get down when told as well.
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.
You will also want to keep a leash on her at all times initially to grab if she should disobey. I think you will be pleasantly surprised at how well your dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
Additionally, I would suggest you get a basket muzzle and make her wear it anytime children are around or visitors are there. Be sure to use the leash to make her obey you. If he growls give a short tug to get herattention and a firm "NO" to let her know, you are not going to allow her aggression. If she is sleeping, give a little tug to let her know that someone is there so she isn't startled when being woken up.
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.
If she growls at specific people regularly have them help you with training. When they approach and she growls, reprimand her with the low toned NO. After a few reprimands she should start realizing what you are reprimanding her for. When she doesn't growl, reward her with the treat and she will quickly learn that the acceptable behavior gets her tasty treats. Between this training and the obedience training, you should see some improvement within just a couple of weeks. Family members should also be involved in training, but I'd start them practicing with her AFTER she knows the commands so that they will see them as her boss as well as you.
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.