Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about your little one Sam's two day history of passing a drop or to of blood after a normal looking stool.
Has he ever had loose stools?
Has he ever had mucous in his stools?
Does he strain to pass stools?
Are his gums a nice pink color?
If his gums are nice and pink, the bleeding is only drops, and since he seems himself otherwise this isn't an emergency. He can wait to see his regular veterinarian this week.
I would recommend that they check his anal glands when they see him.
Anal glands are scent glands located at the 5 & 7 o'clock position around the anus. If for whatever reason the glands don't empty properly or the discharge becomes too thick the gland overfills, may become infected and is painful and we may see drops of blood passed after the stools due to chronic irritation/inflammation. This is a common problem for dogs and seems to be more common in overweight dogs that are fed lots of table scraps or eat a diet low in fiber. These glands are normally expressed when passing stool but if he eats a diet low in fiber his stools may be too small or infrequent to empty them. Dogs with full anal glands may scoot their bottom on the ground, even sometimes spinning in an attempt to empty their glands, or they may lick their bottom excessively. If he cannot reach his bottom very well he may just look at it quite a bit and grumble or in some dogs the only symptom may be a bloody discharge.
In the majority of cases when we see fresh blood in the stool or after passing stools it is related to anal gland problems (infection/impaction or a mass) or inflammation of the lining of the large intestine (colitis) including a possible mass in the colon.
Parasites and/or an overgrowth of abnormal bacteria would be the most likely problems if the blood was seen as a part of the stool. Food allergy or sensitivity would make sense too with blood in the stool. Those pups usually have loose stools and often have mucous in their stools, and they often strain to pass stools.
Very rarely a dog can develop abnormal blood vessels on the surface of the colon that bleed with the irritation created by ingesta moving over it (colonic vascular ectasia). The only way to diagnose this is to scope the colon. At that point if found it can be addressed via laser treatment of the blood vessel(s) if there are only a few areas affected. This should be performed at a referral specialty clinic as it is delicate work. Most of these dogs bleed enough to be anemic, because they have multiple areas that are affected.
Your little one sounds like he is only bleeding a few drops, so this seems less likely, but is worth considering if everything else has been checked and treated.
Please let me know if you have any further questions.