It sounds like colitis......is the stool dark, runny and bloody? Are there any other symptoms? My yorkie has bouts of colitis usually because of something she ate. I have her on a strict diet because of it.
Her coilitis usually flairs up if she has eaten something she wasn't supposed to eat. There are usually no other symptoms and it usually clears up in a few days.
Here's a good article and run it by your vet as well
https://www.petcarerx.com/article/to...s-in-dogs/1514