Housetraining my puppy?

My puppy is a 6 month old rottweiler. He started house training at 12 weeks old, took him about a week to get the idea, but i carried on with the training for a further 3 weeks or so til i was confident he knew to go outside.He still has accidents overnight, but thats fine as i know they cannot hold on for as long as an adult dog, so i kind of expected that.
The problem is, recently he has stopped wanting to go to the toilet outside. He used to whine at the back door til i opened it for him, now he just goes when he needs to, always in the living room by the front door. If i take him outside he does go, but if i dont lead him out, he always goes inside, its as if he is marking in his own home, as a few times he has urinated just after going out to urinate.
This behaviour has come from nowhere, so what should i do? Just continue to housetrain as i was doing before? He isnt neutred yet (but he soon will be) and he is happy and healthy.