Seems to be the thing with new puppies. When we first got Max at 9 weeks old the first week was practically spent down the vet with bloody dirrohea. It was constant, and trailed all over the floor as we couldn't get to him in time to take him out. I was worried sick, the breeder told us he has vaccinated him himself but my vet wanted to be doubly sure so he did it again for the 'records'..he thought it just might be 'stress related' moving to a new home away from his family etc..
One day on his second week home there was just blood everywhere, emergency visit with vets, who were great, they gave me a paste called 'pro-kalin' which I had to give him over the weekend to settle his tummy, I also had to starve him for 24 hours, very difficult to do with a hungry puppy, and re-introduce his kibble slowly - well it worked - his poop began to firm up and honestly we have rarely looked back.
I know they can be ill but sometimes it's perhaps a change in food or circumstances that bring it on. Though Medical advice should be always sought.
Max (Tri Babe) & Bran (The Ruby Man) & Springer Sal (Waiting at the Bridge)