Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that CoCo is eating poorly and seems lethargic.
Is she drinking any more water than usual?
Any weight loss?
Has she had a recent heat cycle?
Are there ticks in your area, and if so is she on any type of prevention?
A decrease in 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, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, internal organ failure, Addison's disease (hypoadrenocorticism) or even infiltrative cancers such as lymphoma.
Tick borne infections can cause intermittent lameness and a low grade fever and lethargy so if ticks are a problem in your area that is worth looking into.
Since she hasn't eaten normally in several days ideally she would see her veterinarian soon. Simple stomach upset should pass within 24 to 48 hours. They could examine her, run blood tests (complete blood count, biochemistry profile and a tick panel) 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 would prefer there are some things you can try at home, but if she isn't bouncing back a hands on examination is best.
At home to try and settle her stomach you can give either:
1) Pepcid ac (famotidine) at a dose of one 10mg tablet per 20 to 40 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of one 20mg tablet per 40 to 80 pounds of body weight every 24 hours
These are acid reducers and may help her feel less nauseous so that she will eat better. 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 meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice, plain potatoes or pasta mixed with some low salt chicken broth to make it easy to lap up and swallow. If she eats this well for several days you can try reintroducing her normal diet slowly by mixing it in, a little more regular and less bland at each meal.
If she continues to not eat well or stops eating well if you try reintroducing her regular dog food 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.