Kayla asked for help with this problem:

“I have a 15 mo English Mastif that has recently decided to start pooping and peeing in our bedroom closet. Their hasn’t been any changes in our home and I don’t know why this has started or how to fix it.
Thanks”

Regardless of what has caused the change in your dog’s behavior, my advice would be to limit where your dog is allowed to roam within the house, at least until the behavior has been sorted out.  Prevent access to the bedroom using a baby gate or a gate specifically designed to confine dogs. Also, it would be best for your dog to be confined either to a crate (properly introduced to it first) or to a single room where a mess is not going to be too much hassel for you to clean up if he has an accident.  He should also be less inclined to soil this area, or at least limit it to one small area.

Remember that you need to keep your cool if you discover a mess.  It could be that he is sneaking off because he has been punished previously after messing in the house.

What you really need to do is try and predict when your dog needs to go, so that you make sure he is in the right place at the right time.  This will mean that you need to be close by him whenever he is not confined. If you do not already have a feeding plan, take advice from your vet, or buy one of the recommended housetraining guides that contain sample plans which you can use based on your lifestyle and dog’s size and age. At 15 months, your dog will probably be able to be limited to two meals per day (again, double check with your vet or other animal diet professional first), so if you feed to a strict timetable, it should be easy for you to have a good guess at when he needs to poop and take him outdoors before he finds a secret place to leave you a little present.

Once you both get back into a consistent feeding/watering routine, your Mastiff should start being clean in the house once more