Such a weight gain in a short time with a poor appetite and extreme lethargy is concerning. She could have a metabolic problem such as an underactive thyroid. She might be retaining water, which could be from a variety of conditions, including heart and liver problems. She needs to be evaluated by a doctor to find out what is causing these problems before any medication can be prescribed. A medicine used in one condition might be the worst possible choice in another condition. So it is important to know the cause and what the specific treatment would be. Hope this answers your query. If you have more questions, I would be happy to answer them.