Friend, vomiting and lethargy can be caused by either primary disorders affecting the gastrointestinal tract or secondary to a systemic disease. The fact that she is drinking more than usual we need to rule out things like: Pancreatitis, ingestion of a foreign body, kidney or liver disease, hormonal disorders, tumors, etc. to name a few causes.
In this case, I would strongly recommend having her re-checked by family veterinarian. Discuss with your vet about running some blood work, urinalysis and xrays as initial screening test to rule out the above mentioned diseases. In the meantime this is what you can do:
1- Do not give things like Pepto Bismol or imodium. Start her on Pepcid AC (famotidine) 10mg 1/4 tablet every 12 hours for the next 2-3 days. You can buy Pepcid AC over the counter in any pharmacy. Do not start her on Flagyl until she is re-checked by family vet.
2- No food or water for 6-8 hours. Need to give her gastrointestinal tract time to rest.
3- After 6-8 hours offer a small amount of water. Wait half hour, if no vomiting, offer small amount of rice (50%) and boiled boneless chicken or ground beef (50%) 4-6 times a day.
4- Follow up with your family veterinarian as discussed above.
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