Dog Peeing on Herself When Lying Down?

My dog is a Min Pin. This is the third time she has peed on herself in the past 6 months. She doesn't even notice at all when she does pee. She will just lay in it like it's not there. She is just 6 years old so she shouldn't be having these problems yet? She has been spayed, I'm not sure if that could be the cause?