Icks, I just had this with Jack/ they (vet) told us he might have gotten in some Bad water, Come to find out our pool and pond had bad water (mold) and it was Really Bad but she gave him a shot and kept him for the day. that night he came home and no more bloody stools.
It was intestional she said. My husband drained the pool power washed and put new water and drained pond. A week later I took him in for check-up. She told me he Looked very good not a sick puppy anymore. In fact she and everyone remembered that he was not the same puppy they had seen before.
I cried for hours when we were going through this.
Cheryl/Jack/Bella