My dog is all of a sudden pee on the carpet?

She is a Beagle that is almost 2 years old. She is potty trained, and is now all of a sudden going inside the house, with my thoughts to even go outside. She was at the Vet on Friday, so she is not sick. The problem is 9 times out of 10 we don't catch her. But today she was playing, stopped and just starting to pee right on the carpet in front of us. What do I do?