Trained Puppy having accidents

My perfect 6 month old maltese who NEVER had an accident in the house for the last 3 days has decided to go poopie in the house and not in his run,,,he still goes there for pees but waits to sneak off in the morning to poop in the house...and clearly knows it is wrong by his demeanor when he reappears with ears back and tail down.I will now cage him in the am till this habit is overcome but i am mystified by the reason behind this behavior.Any guess as to his thinking??

[Aleda M Cheng, D.V.M., C.V.A]

Before I would completely blame this on behavior I would have his stool checked.  Have your veterinarian do a regular fecal test and a test for protozoa called Giardia Lambia.  Your vet may have to send out the stool to a lab for the Giardia test.

Even if your pet has already been tested and wormed he could still have the parasites in his system either by never having them completely eliminated or by re-exposure.  Parasites are extremely common in the environment, and since parasites life cycles vary it may take many attempts before they are detected.

I would also have your vet check your pet