I Have a 5 month old puppie. A day ago one of his back teeth fell out when he was eating his food and he swallowed it and now there is a little blood in his poop.

Normally when they swallow their teeth, it's not a big problem. The baby teeth are so small that they almost always get swallowed and pass with no issues. However blood in the stool can mean many different things, which can only be diagnosed by a veternarian.
Is your puupy up to date with vaccination?
Honestly Melissa, there are just too many possibilities to list. Including damage to the esophagus or stomach lining.
What color blood, and how often is there blood in the stoole?

Thousands of pups all over the world just did the same thing your dog did with no problems. When ours are teething we only find one or two on the floor, the rest get swallowed!
However, like Aurora said it may have pierced something, due to the blood in his stool.