HGE or Hemorrhagic Gastro Enteritis:

In really small dogs but especially YORKIES and Chihuahuas - they are very vulnerable to infections in the COLON and so you see 'diarrhea' first - and then sometimes it "goes bloody" and the colon starts peeling.
This is called "Hemorrhagic GastroEnteritis" which is just blood from the colon.
This can get serious and dehydration can simply kill 'em because the dogs "hardest hit" by this are under ten pounds and don't even hold much water.
If you can get 'em to me before they are super dehydrated, and while they're moving around and active "like nothing's going on" - then they usually do great.
If they're pooping chunks of red/purple slime and depressed, and dehydrated most of the time it's a good idea to put 'em in the hospital.
For now, we can use a battery of medications and LEAVE THE BOWEL EMPTY for twelve to twenty four hours to heal - and maybe some fluids -> and USUALLY no hospitalization or emergency clinics and neuro exams are needed.
Suspend food 12-24 hours
Give medications per instructions
Encourage drinking water or even Pedialyte/Gatorade.
Report depression, blood chunks in poop, loss of skin-flexibility.
Doc Johnson