My dog pee,s in the bed. Why?

I have had a Terrier type dog for about three years, I adopted her after someone dumped her. Best i can tell she is about 5 years old. Very dependable on going outside to do her business until recently, she sleeps on the foot of the bed and she has been wetting the bed, at first i thought she was mad at me but i saw her today asleep on the couch and she peed all over herself. I have never heard of this. Has anyone else? What would make her do this?