Diarrhea in dogs is very common and it can be caused by many factors.  Here is an article to help understand the differences and if it is something to worry about:  https://www.vetbabble.com/dogs/health-dogs/diarrhea-in-dogs-when-to-worry/

Here is another article which has a bit more detail on the types of diarrhea your dog may be having and what be causing it:  https://www.vetbabble.com/dogs/questions-dogs/why-my-dog-has-diarrhea/

However, in your case, you mention that it has been going on for a few days.  This is troubling, regardless of the case and if has not begun to show signs of improvement, you should definitely visit a veterinary office to assess what is going on, possibly run an exam to look for any parasites and start any medications or treatments if necessary.