Puppy Pooping on Wife's Mattress?

My wife and I sleep with the mattress on the floor at night. We have two separate single size mattresses combined (while waiting for our new home). My 4 month old Shih Tzu is potty trained. She pooped in her cage in the room or the pee pad, but recently, she started pooping on my wife's bed.

The moment I changed the bedsheet, she peed on it. Just this morning, I woke up in horror to see my wife covered in Pancake's poop, still sound asleep. She only does that on my wife's mattress and not mine. I don't think it's my food supply because I haven't change it for a while. Her cage is opened for her to do her business all the time, ever since she was potty trained, but why now? Any advice? Is it because she can't hold her bowels because she is only 4 months old? But she only did it wrongly on my wife's bed and not else where. When the mattress is lifted up, she will pee and poo in the cage. I need help here!