Hello, my name is***** and I have over 20 years of experience as a veterinarian. It sounds like Bella has a poor appetite and minimal water consumption and has been lethargic today.
Lack of an appetite and lethargy can be related to eating something she should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), chronic pancreatitis, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, Addison's disease (hypoadrenocorticism), inflammatory bowel disease, or even infiltrative cancers such as lymphoma. If she is not spayed and has recently (within the past month or two) had a heat then a uterine infection called a pyometra is possible too.
Because she hasn't eaten or drank normally today I am very concerned about her, especially because she is lethargic too. If it is very warm where you are if she isn't drinking well either she will quickly become dehydrated.
Ideally she would see a veterinarian tomorrow if this continues. Simple stomach upset should pass within 24 to 48 hours. They could examine her, run blood tests and possibly check radiographs and/or an ultrasound to evaluate her and know best how to treat her.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate her.
If you cannot have her seen for whatever reason there are some things you can try at home.
At home tonight to try and settle her stomach you can give either:
1) Pepcid ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one quarter of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours
These are acid reducers and may help her feel less nauseous so that she will eat and drink. They are quite safe and can be used for several days if necessary.
A couple of hours after giving the acid reducer I recommend offering small amounts of water or clear broth. If that goes well leter tonight or tomorrow morning offer her meat baby foods or a bland diet of 1/3 boiled, minced, white skinless chicken or boiled, lean hamburger and 2/3 boiled white rice mixed with some low salt chicken or beef broth to increase palatability and fluid intake.
If she continues to not eat she should see her veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.