It is hard to say what is happening. You may need to keep a close eye on her being quick to let her out if she needs to go if she does have a physical problem.
At 3 years old, her personality is in the final stage of maturing. As they mature, they sometimes challenge your status including by eliminating in the house. If you haven't obedience trained her, or did not follow a good program, now might be a good time. The dogs see all the
people and dogs in the household as a pack with each having their own rank in
the pack and a top dog. Life is much easier if the 2 legged pack members
outrank the 4 legged ones. You can learn to play the role of top dog by
reading some books or going to a good obedience class. A good obedience class
or book is about you being top dog, not about rewarding standard commands with
a treat. Start at
http://www.dogsbestfriend.com/.