old dog new tricks

Hi all, we have been sooo lucky for so long. Dusty was house trained, I say was because for the past month she has toileted in her den almost every day and I sadly am getting very frustrated. We have not changed her food but did increase it a little as she looked a little thin but not a huge portion maybe 15% more. We took it back to what she was having. She is fed twice a day, about seven and again at about five. She has a smaller portion in the morning than the night. Do I have to now go back to the start? should I change how and when I feed her? Any help appreciated. PS she is one year old now. Thanks for reading  

Have you taken her to see the vet in case it is anything else ?

How long after she is fed do you notice that she has done it ? Does she do it the same time every day ?

Darcy is 11 months and a couple of weeks ago she started weeing on the upstairs landing! She'd never pee'd there before ever but it was like she forgot she goes out side I just went back to basics and started again with potty training, putting her out every 1/2 hour or so and after a week or so she was back on track, as Kathy it could be something the vet visit could help with, what are you using to clean up the mess?