I'd go to the vet and get a check for parvo, dear. Even dogs and puppies that have shots can get it. Don't freak out because if they had their shots that's probably not it, but the vet can help pinpoint what's wrong.
In the meantime I'd suggest a rice, banana, and pumpkin mix to give high fiber and lots of nutrients to help balance the system.
Did you change foods recently? Is there any way they got into trash or something else?
And, might sound strange, but what kind of diarrhea was it? Clear, gel-like, just typical runny poops? That tells a lot too.