You should call her doctor. Some people are allergic to amoxicillin/penicillin. Also, the amoxicillin wouldn't prevent her from contracting a virus. So, she also may have contracted a stomach virus.
If you look back on old posts, you'll see one from me this past summer regarding bizarre vomiting and diarrhea with regular behavior from both of my boys over several weeks. The doctor said it was a virus that was taking a long time to get out of their systems. All of my experience with stomach viruses has involved the violent 24-48 hour nightmare kind of constant nausea, vomiting and diarrhea that makes you plead for death! I had no experience with what was going on with my kids. They would act and feel fine -then one had diarrhea for 10 days, but acted like he felt great and the other one vomited every other night for about a week, then had diarrhea, then vomited a few more times on random days! The youngest went through the diarrhea for 10 days then went through a week of vomiting once a night or every other night. It was crazy! Keep pouring on the fluids, but speak to her doctor.