Diarrhea problems - Does anyone know why?

It could be the stomach flu, or just a stomach bug or virus. It could also be food poisoning. Diarrhea can be caused by many different things. Sometimes it's just a virus, other times it's an indication that something else is wrong. It can also be caused by digestive problems like IBS, Crohn's disease, etc. I would just monitor how long it goes on. If it goes on for too long, then she needs to see a doctor, and they will know what to do.