Ugh, we are having the SAME problem
Dexter is 4.5 months old. And at first he was getting potty trained just fine, we never taught him to pee on the pads (although we still have them randomly around the house), we taught him to go outside. But few weeks ago he started going inside! He knows he is not suppose to cause he hides it and looks VERY guilty after he's done the deed. He can go whole day without peeing while we are at work, and doesn't wake us up at night anymore, so there is no problem with holding! I try to take him out every hour when we are home in the evening, but he still would pee inside! What do we do??? :'(