My 8 week old puppy diarrhea. why?

Ive had my dog for about 10 days now. He is 8 weeks old. I never got a chance to ask the previous owner what kind of food he was eating but as soon as I brought the little guy inside he went straight for the dog food that my older dog eats. He and the older dog have been eating Purina 1. And I asked a local breeder if it was bad for the puppy to be eating adult dog food so early. He said there is not to much difference in adult food vs puppy food so he recommended just giving the puppy a little at a time. So for about 6 days he was fine. His potty looked normal and his gums were healthy. But the past 4 days his Business had blood in it and was starting to get runny. I am wondering what could be causing this. I figured it could have been the change in food but why the blood? I have scheduled a vet appointment already but I wanted peoples opinions on this while I wait to go to the vets. And if it is the food change.. is it normal to have a delayed reaction towards it? That and could stress be an issue? I think he looks fine. He has been playing and running around and absolutely loves my older dog. Crate training him though, has been difficult since the diarrhea and of course I'm not going to let him roam around the house and poop ever wheres. What a mess that would be. I don't see any worms in his poop either. I know this is something for a professional to handle but I wanted to see what everyone else thought while I wait to take my puppy to the vets.