Diarrhea can be due to stress, a quick change in diet, eating unusual rich foods, parasites, inflammatory bowel disease or even a food allergy. It can also be related to metabolic organ disease (such kidney or liver disease) due to organ wastes irritating the gut or a mass in the intestinal tract.
If your pup has had diarrhea for several days and he is straining to pass stools that can lead to enough irritation to cause bleeding. As long as the blood is a small amount, and his color remains good (gums a nice bubblegum pink) then I would not be too concerned about a little blood.
How is he feeling otherwise?
Is it possible that he was fed something different or too much table food at Thanksgiving? If so that may have started all of this.
If he is feeling relatively well otherwise I am less concerned. If not he should be checked by his regular veterinarian if he's not bouncing back quickly with the suggestions that I give you.
Well worth checking at least a couple fresh stool samples too as parasites are a common cause of loose stools in dogs and parasite eggs/cysts are shed intermittently. They may be picked up on the second sample or third test even if the first is negative.
If he still feels well now though (no vomiting and still wiling to eat) we can try some things at home.
I don't normally recommend using Imodium or Lomotil in dogs as it works by slowing the gastrointestinal tract down so that water in the stool can be reabsorbed. It is a narcotic and can lead to sedation and in some dogs can stop normal gut movements entirely. If the diarrhea is related to a bacterial toxin the Imodium allows more toxin to be absorbed.
If his appetite is less and if his stomach is gurgling he may have some stomach upset and reflux that can go along with loose stools. To try and settle his stomach today you can give either:
1) Pepcid-ac (famotidine) at a dose of one quarter of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one quarter of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These will reduce stomach acid and should help if this is related to simple nausea and gastrointestinal irritation from eating a large amount of rich food. They can be given for several days if need be. If his appetite seems fine then you can skip this step.
I would not feed him any food for 24 hours. This should help stop gut spasms and restore normal gut motility. Small amounts of water or ice cubes given frequently are fine as he needs fluids after all that he has lost with diarrhea. You can give him pedialyte to replace electrolytes too but Gatorade is much too high in sugar which can make his intestinal irritation worse.
Today even with the fast you can start Kao-pectate at 1/2ml per pound of body weight or 1/2 teaspoon per 5 pounds of body weight every 8-12 hours. This will coat his irritated gastrointestinal tract as well as absorbing bacterial toxins. You can use it for several days until his stools look normal. You can find kao-pectate at the drug store.
If he has a tense painful abdomen, continues to have diarrhea with no improvement after being on kao pectate for 24 to 48 hours, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then he really must be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
After his food fast start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken), all fats and juices drained off the meat, mixed with 2/3 boiled, plain white rice. Feed several small meals a day. You might wish to add 1/2 to 1 tablespoon of canned pumpkin to each meal if you are seeing mucous in his stool as fiber helps soothe an irritated colon.
You can also add probiotics to his bland diet mix to replace the good bacteria lost with diarrhea. Good brand name products include Fortiflora, Proviable, or Benebac. They can be found online or at pet stores.
Once he feels better (no diarrhea for 48 hours) start mixing in his regular dog food very slowly, less bland more regular with each day. It should take a week or so to convert him back.
If he begins vomiting and feeling poorly though it would be best that he see a veterinarian now as anything you give him orally will just come back up worsening his dehydration.
Please let me know if you have any further questions.