Can a pup have blood in is poop by changing his /her food?

No, just changing the food would not cause that.
If you changed the food suddenly it can cause diarrhoea or constipation, not bleeding.

Time to take the pup to a vet to find out what's going on.

How long have you had the puppy? Sometimes puppy can get blood in their poo because of stress. If their poop smells abnormally horrible you could have something bad going on in there. Also, bloody poop can be caused by massive amounts of worms.

Might want to get a fecal done just to be sure