I have a niece that is 20 years old and has bouts with diarrhea for 1 or even 2 weeks at a time. What would causes for this?

No way to know! This sounds serious and I would recommend she start with her pcp and perhaps see a gastroenterologist and have further testing done. She could have repeated infectious diseases, an inflammatory bowel disorder, an irritable bowel and many other conditions that should be evaluated and treated. Encourage her to be seen!