My wife and I have a 3yr old male Amstaff. He truely is the best dog as we have heard from other people. Anyway with in the last 2-3 months he has been marking in the house. Either when we are at work or at night. He goes out to do his business regularly so it's not like he has to go, he's just lifting his leg to mark. He has never done this in the past, but now it's almost an everyday thing. what is going on?? Please help, we've had it!!!