There shouldn't be any cause for concern. A small amount of blood in the stool ('poop') is normal for a recently spayed dog.
If you think there is an unusual amount of blood, see your vet as soon as possible.
What you should do if your spayed dog is pooping blood
To minimise bleeding, you should feed your pet a bland diet (boiled white rice and chicken breast), and make sure they are drinking enough water. Of course, before undertaking any medical advice from the internet, you should attempt to see a veterinarian first. To keep costs to a minimum, call the surgery first - advice is free, an exam isn't.
So unless you feel that the blood is excessive, you should start her on the diet recommended and make sure she drinks enough water.
I hope she gets better soon.
Dogs are usually stressed post-op, so make sure she's in a calm and quiet environment. If you do decide to see the vet, they will most likely give her something to settle her stomach, and advise the same diet.
If anything goes wrong, call the vet immediately.