Housetraining going backwards

I just don't understand it. Riley went #1 and #2 inside AGAIN today. I just don't understand what I'm doing wrong w/ the housetraining. He WAS doing so well, it like he's going BACKWARDS now. He's 13 weeks and up until now he's HELD his bowels and bladder so well until outside (for the most part). I take him out every 2 or 3 hours. I take him out when we get up. Then I feed him. Then a little while after I feed him I take him out again. Then before I leave for afternoon errands, I take him out again. I did this today. I was only gone for 2 hours (and last night he held it over night for 10 hours, he went #1 AND #2 in the morning. So even tho he went out several times in the am and well after his breakfast, he pooed in his pen while I was gone (the pen is folded down to the size of a small crate, just enough room for a bed and bowls) and then RIGHT when I got home he peed right in front of me in his folded down pen without crying or whimpering or anything.

I just don't understand. I know not to expect perfect housetraining at this point, yes, I know he's a puppy, but he WAS doing so well it's like he's going -backwards-. That's why I'm so confused.