My boyfriend had an 11 month old beagle puppy that started having diarrhea that was nothing but blood either late Friday night or early Saturday morning. He called the on call vet and was rudely told to wait until Monday to bring him in, that a puppy throwing up and having all bloody diarrhea wasn't an emergency. Then when I got up today, Sunday, I found him dead. I was just wondering if you could tell me what may have caused this to happen.

Oh gosh, I'm so sorry you and your boyfriend had to deal with this.
I would be having very strong words with a vet who would turn away a pup with those symptoms.

Unfortunately there are many possibilities as to what could have been wrong, everything from poisoning to a blockage to parvo. You could have a necropsy done to find out exactly what happened (which would probably be a good idea if you have other dogs) but other than that it's all just guess work sorry.

I am so sorry to hear about your poor little puppy. My best guess is on parvo. If your pup wasn't vaccinated then chances are good that's what caused his untimely demise. It is the leading puppy killer and once the pup starts showing signs, they usually go from bad to worse in a matter of hours. I agree with Shazzy to get the autopsy done. ESPECIALLY if you have other dogs or pups in the house. If it is Parvo, you will have to do an entire cleanse of the house with bleach.