Why would my dog suddenly start pooping and peeing in the house, on the carpet, after over a year of pretty much flawless behavior?

Lizzie is 14 months old, I got her at 2 months and she was almost effortless to house train. The only issue was that she would whimper both when she needed to go out and when she was feeling alone.

But for over a year she has not had any problems, just all of a sudden.

Even as a puppy she had the bowel and bladder control of a champion. The first night I had her, in the hotel, on the way back from Indiana, she didn’t have an accident. Except for a couple of peeing incidents, the only “accident” she had was during a thunder storm when she literally had the sh*t scared out of her.

She sleeps with me, all night, with the bedroom door shut so she doesn’t get into the rest of the house and eat things, and except for the thunderstorm incident, has never had an “accident” overnight.

What’s going on with her? There were no massive changes in her life or environment, as far as I know, when this started happening.