My advice is do take your dog
to the vet as soon as possible. There is a stool test for
Parvo which can tell quite quickly, if it is this virus or not.
Parvo does usually present with a fever, lethargy, vomiting, and very watery diarrhea usually with blood . But even though your dog doesn't seem too bad otherwise, there is no harm in being certain. If it IS just a very badly upset belly, well, you will have erred on the safe side.