Annie had the same problem once... Vet said she must have just gotten into something she shouldn't have... Carpet, paper etc... I was really surprised myself cause she hasn gotten into things she shouldn't plenty of times and usually its just vomitting or diarhea. But this one time it was really bloody stools. So I was really worried. But he gave her some antibiotics and then she was fine...
So I hope that is all that is wrong with Bentley. I bet he will be back to himself tomorrow!