Nah I don't think the only real answer is Parvo mate to be honest, by the sounds of it I'd bet it was the change of food and bones. I had a similar thing happen with a pup I have, I gave it one of these roasted knuckle bones from the pet shop and his shits were runnier than normal with a bit of blood in it. Just turned out that it was the bone that I had give him.
From what I've read, dogs tend to go lethargic and off their food before they start shitting blood when they have Parvo, where as your bitch is the opposite.
Still inform the vet mate, as you never know it could be something