Dog Started Pooping Inside?

My Charlie has just started this really bad habit of pooping in his room. He is 6.5 year old Labradinger. We walk him 3-4 times a day and 1 time to the dog park. He has only just started doing this and as I am the one cleaning it, I know it is going down right after we put him in the room as it is cold when I clean it. I aquired him at 2.5 years old so I have had him for 4 years now and this is the first he has done this for 3 days in a row.

He was an outdoor dog up to about 4 months ago when he was shipped here to bring our family together. I thought he had adjusted to apartment living, and there were only a few accidents the first few days, then it stopped. I am a nurse and my shifts change from days to nights every now and then, if this is the problem, then why would he be doing this on my days off? I am at a loss for a reason he is doing this. How can I make it stop?