Usually with the smaller dogs, their butt area could become irritated, this is because the anal glands are inflamed, you should take your dog to the vet to have it squeezed. This is what I had to do with my maltese poodle. Good luck.
Why Does My Schnauzer Lick His Butt Often At Times,its Like It Itches Him?
I don't know, but it might be the food he/she is eating. My dog does the same thing and I checked his poop for worms and didn't find any. The vet says to bring them a sample and they can check it.