It does sound like the stomach flu. She may have had a fever that you missed or might not have one at all. If she had more of a serious condition, it is likely that she would be in constant pain. The flu strains that my family has experienced, as well as I have seen in other families lately, is not an easy on one to get rid of, and it can keep coming back when you think everything is ok.
If you are still having problems like this after a month, then you will have reason to be concerned, but I really do think that it is a stomach virus. Keep her well hydrated; the end of it will come about in time