Agree with Ghilly but, esp due to her age, I would definitely be calling the Vet to have her examined. I would really be worrying about Parvo. Did she have any shots prior to you getting her?
Depending on what kind of food you gave her, it COULD just be that she overindulged and her system is rebelling against the heavy food intake, however just to be on the safe side, I would make an appointment with the vet and take her in to be examined. Most of the time if it is simply a digestive disturbance from overeating they will not act TOO much differently than they normally would. They'll have diarrhea for a day or two and then it will go away and they'll be back to normal in the bowel movement department. If she is vomiting and having diarrhea and acting weak and very low on energy, then I am thinking there may be something bacterial going on. Diarrhea and even occasional vomiting from overeating won't usually cause them to become lethargic and act really weak. Depending on whether or not her vaccinations are up to date it could be the beginning of something serious, so it's important to get her checked out to make sure it's not the early stages of Parvo or something equally as serious.
Please let us know how she is!
Ghilly