Bloody stools can be very frightening for dog owners. Our current dog has had them when she has reacted to pain medications as well as when she has had some serious diarrhea. But with our first dog, it was the sign of something serious. Definitely call your vet.
If it isn't something serious--but more like gastroenteritis--then they will probably have you put her on a low-residue diet and gradually work back to regular food.