Blood in the stool means that the gastrointestinal system is so irritated it is raw. If her colon is affected then bright red blood will be seen, the darker the blood the more digested it is and the farther up the gastrointestinal tract it is from.
This can be from a bacterial or viral infection, eating something she should not have (toys, bones, bugs etc), gastrointestinal parasites, a dietary allergy or sensitivity, and a quick change in diet.
If she were older inflammatory bowel disease or cancer are other possibilities, but not in a puppy this young.
If she has not been vaccinated or wormed viral infections and parasites are a common cause in puppies. If she is lethargic at all, and/or is vomiting then an emergency clinic visit now is best. Chances that this is something serious like parvo virus are much higher, and that should be aggressively treated.
If she still feels well otherwise though we can try some things at home.
I would not feed her any food for 6 hours. This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as she needs fluids after all that she has lost with bleeding and diarrhea.
Today even with the fast you can start Kao-pectate at 1/2ml per pound or 1/2 tablespoon per 15 pounds every 8 hours. This will coat her irritated gi tract as well as absorbing bacterial toxins. You can find kao-pectate at the drug store.
If she has a tense painful abdomen, continues to have diarrhea with no improvement over 24 hours even with kao pectate, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then she really must be seen by a veterinarian as soon as possible.
After 6 hours start a bland diet of 1/2 boiled hamburger (or white skinless chicken), all fats and juices drained off mixed with 1/2 boiled, plain white rice. Feed several small meals a day.
Once she feels better (no bloody diarrhea for 48 hours) start mixing in regular dog food very slowly. Less bland more regular with each day. It should take a week or so to convert her back.
You should submit a fresh stool to her veterinarian for a parasite check. Parasites are very common in puppies. And if she hasn't been vaccinated fully she really should be seen to get started on her vaccines and for a thorough physical examination.
Please let me know if you have any further questions.