Bright red blood in stool. not on toilet paper. no other symptoms and haven't ate anything red recently. 16 yo female. cause for concern?

Yes: Bright red blood per rectum is always a cause for concern. There are many causes but but the underlying cause needs to be found and soon

Bloody stools: Blood in stool is always problematic. While it is often an anal fissure, hemorrhoid or polyp in the intestines, you need to see a physician to determine what is causing intestinal bleeding. It can also be a sign of cancer or inflammatory bowel disease. You need an urgent physical exam by a physician to find the source of the bleeding.