You may be offline for the day and I have to soon go test some dogs for therapy work. I will try to answer you going by what you have written in.
Of course the concern would be a potential health problem, but you have ruled that out so let's look at behavior.
If there was company over or another animal visited this could cause the dog to go in the house suddenly.
If she initially had an accident and was scolded or anger came out in body language when the human was cleaning up after her then That can have an impression on them and they will try to sneak off from the owner and potty in the home.
Doggy doors are okay , but if something has frightened the dog when it used the doggy door then that is what they begin to associate that door with.
Though we don't know for certain why the backslide, then it is time to retrain in a positive way and make her memory of going outside one of pleasure so that she will want to continue to do so.
The first rule would be to never allow her out of sight inside the home. To do this is setting her up to fail. Kennel her or tether her to you, or be sure she is at least in the same room with you at all times.
Secondly take her out often on a leash and teach her a potty command such as 'go potty' and continue to give this even as she is going, then reward her with something special such as a hot dog sliver. Eventually by doing this you can have her go on command at any time.
The doggy door may be the problem so it is possible you have to have her associate that with something good again as well. Once you have her going potty on command then start lifting the doggy door and give the command for her to go potty. If she is afraid to go through it , you will then know it is the door itself she is afraid of. She may have gotten hurt going through to slow, a loud noise may have gone off as she was going through, almost anything could have happened without your knowledge. Then it is time to make that a pleasant way to go out again. To do this you can use it to play games by tossing her favorite toy through it or a treat. Anything that is positive for her.
Let me know if you do not understand something or if there is something that "I"misunderstood and I will get back to you as soon as I can.
If you are satisfied with my answer please do not forget to rate so that I will be compensated for my time .