This have happened to my Yorki a lot of times. In the beginning I went crazy with worry and was running to the vet every time it happened but all the vets I took her too said to relax. A small puppy have such a small stomach every thing its eats have an immediate effect and can irritate the digestive system and it eats everything! It find things on the floor in your house that you dropped cooking or eating months ago or that are not for eating in the first place
Every time a small puppy eats is actually quite a big quantity for a very small fragile stomach and that might result in small amount of blood in the stool. So unless the puppy throws up as well and is drowsy and refuse eating- don't worry!!!
If it doesn't pass in a week go to see a vet otherwise expect this to happen every now and then.