Is it because I'm changing his diet?

No, a sudden change in diet can cause digestive upsets, but not blood in the stool. The puppy needs to see a vet. With no other problems, Monday should be soon enough. Lethargy and a foul smelling, bloody diarrhea can mean parvo. Parvo means get the puppy on an IV as soon as possible for any chance of saving it.

Please let us know how it does, and more details.

Also, see the sticky starting at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post251802

Yes, please listen to labman, and get your pup to a vet asap.