[F]my Puppy is 3 months and we just got her yesterday and now were discovering blood in her stool.
What should I do?

Sounds like hookworms. Take puppy to vet to get dewormed. The hookworms literally suck the blood from the animal's intestines and can result in your puppy's ultimate death if not treated. It would take a while for this to happen though, but in the meantime the puppy will start to show more and more signs of lethargy.

Yes, it very well may be hookworms, but just in case it may be more serious are there any other symptoms like vomiting? Can the puppy hold down food and or water? Is the puppy lethargic? Has the puppy had all of its vaccinations?