My dog vomits bile reasonably regularly, but really only when he hasn’t got any food in his stomach. However he only has diarrhoea when there’s something wrong, and the presence of blood is a bit more concerning. It might depend on where you live as to what your vet would advise but I would at least give them a ring and see what they suggest. It could be that your dog has eaten something on her walk that she shouldn’t have and has picked up a bug. She’s not on any non-steroidal anti-inflammatory medication is she?