When I was asked why I stopped in the middle of working on my second novel to write s short story, the question surprised me. I hadn't really thought of it as stopping. The story idea was in my head, but it belonged in my character's life many years before the book began. I could have let it remain part of the character's back story, a few details that I knew about her and might never share with anyone.
When I'm working on a story, any story, my brain is puzzling over details all the time. Taking the time to write a novel doesn't mean there isn't another story cooking in the back of my mind at the same time. Sometimes the new ideas that pop up need to be written into their own story. It isn't a break. It is a trip of discovery that makes the character more alive inside my head.
Our minds are wonderfully complex. There's lots of room to explore a panorama of ideas. Sometimes the thoughts are all jumbled together in ways that make it impossible to untangle them and what ends up on the page isn't worthy of sharing with anyone. At the darnedest moments all reason escapes my head. I end up starring at a blank screen unable to write a single word. It is all part of the process.