Hello, my name is***** and I have over 20 years of experience as a veterinarian.
I am very sorry to hear about your pup's lethargy after her episode of vomiting her food two days ago, and now lack of appetite, and refusal to drink water.
Possible causes of nausea, lack of appetite and lethargy include a change in diet, dietary sensitivities or allergies, viral or bacterial infections, or eating things that they should not like too many fatty table scraps or garbage, bones, foreign material etc.
Addison's which is a poorly functioning adrenal gland is another possibility for waxing and waning vomiting and nausea. Metabolic organ failures (kidney or liver disease), pancreatitis, inflammatory bowel disease or even infiltrative cancers are possible causes as well but very unlikely if she is young.
Does she have a history of shredding things or eating anything she should not have recently (toy pieces, bones, garbage)?
Any changes in food or treats?
Ideally she would see a veterinarian today since this has been going on for more than 48 hours. 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.
If you cannot have her examined today there are some things that you can try, but with her history I am worried about a gastrointestinal obstruction and those should be dealt with promptly.
You can give her an acid reducer to try and settle her stomach. 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 half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and may help settle her stomach. These can be used for several days if necessary as they are very safe.
I'd also pick up her food and water for now. A couple hours after one of the acid reducers you can offer small amounts of water, clear broth, or ice cubes to lick.
No food for a few hours. Small amounts of water only.
After her food fast then start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice or well cooked plain pasta. Give small meals several times a day. Feed the bland diet for several days, then start mixing in her regular diet and slowly convert her back.
If her nausea and abdominal pain continues then she needs to see a veterinarian for hands on care, including an examination, possible radiographs, and injectable medication from his veterinarian to get her stomach upset under control as well as fluids.
So watch her closely for continued symptoms even with the acid reducers, blood in her stool or vomit or a fever (more than 103.5F rectally), a tense painful belly or a continued lack of appetite after her food fast and giving an acid reducer. If any of those occur it is time to seek hands on veterinary care.
Please let me know if you have any further questions.