The most common reason for blood in the stool is parasites such as worms or a protozoan infection such as coccidia or giardia. The most common worms found in dogs are: Roundworms, Hookworms, Tapeworms, and Whipworms and Heartworms
In some cases these worms are passed from the mother to the puppy, in other cases such as tapeworms, they are contracted by other methods such as ingesting an infected flea. Most over the counter medicine will only kill one or two types of worms, so your best bet is to take a fecal sample to your vet to determine the type of worms your dog may have and then decide on treatment. Dogs should be checked for worms periodically even if they appear healthy. This is so if your pet does become ill, you are not fighting a worm infestation along with a more serious problem. I know you have dewormed your pup but you need to use a dewormer that covers more than just roundworms and whipworms.
For additional information about worms, see this site
Common dewormers are Panacur, which is a prescription product so you need to obtain it from your Vet and it works on roundworms and whipworms. Wormers labeled as Pyrantel Pamoate are the best over the counter medication for roundworms. I suspect it is this type of wormer that you have been using since it is the kind that is commonly on the shelves of your various stores. Drontal plus is also a prescription product and is used for the treatment of Tapeworms, though dewormers with praziquantel work well against tapeworms. These dewormers can be purchased over the counter, you just need to search the ingredients. Many heartworm preventatives also help control whipworms and roundworms. .
Coccidia manifests as mild or severe diarrhea depending on how infected they are. If it is severe, there may be blood and mucous in the stool along with voming and loss of appetitie leading to dehydration You can read about it here:
Information on giardia
Symptoms of giardia are diarrhea with possible terrible smell, mucousy stool and sometimes bouts of vomiting.
Other causes could be anal growths, infected anal glands and inflammation within the large intestine. You can read about the various cause of red blood in the stool here:
Now it may be that your pup has tapeworms and you have been deworming for roundworms, or your pup could have coccidia which is common in puppies or giardia and finally, if you have been changing dog food or giving your pup table scraps or various treats, that can also cause a dog to have diarrhea with a touch of blood. The simplist thing would be to drop off a stool sample to the vet and have them check for worms and protozoan infections and stopping any food changes and unusual treats.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .