I think sometimes you can overplay pieces....or just practise to much...
I've been given advice when this happens just to not play for a day or two, and then go back to it. I, however, can't bear to take this advice because I'm always like "No! I HAVE TO GET IT RIGHT!!" coz I'm a bit of a perfectionist. But whenever I have taken the advice it has actually worked

It can also be down to stress and stuff....I know that when I play and do something wrong, it really annoys me, and the more annoyed I get, the more I do wrong!!! So that's the time to have a little break

Good luck, and don't worry you'll be back to your usual standard soon

Laura