If your story jumps around in your head when you're not at your PC, that's good news. In the bath, in the car, on a walk, even while watching TV or a movie, and annoyingly in bed when you're trying to sleep - it can start leaping about and going places. Let it run - it's all good stuff.
But soon as you can, get a notebook and jot down the general points. Otherwise you may forget. Believe me, you may. I've lost more plot points by not writing it down than I care to remember now.
Usually I find when the plot starts rolling like this, it hasn't got anything much to do with the bit I'm writing at the moment. That doesn't matter. Get it into a document and let it sit there. It might even be a different book altogether, get it down. It's still fodder for your writing.
When you eventually come to use the ideas, they may have evolved or you may change them to suit. But most ideas end up usable at some point. Years later sometimes.
My historical crime series evolved from an original idea I had about 15 years ago for a "big" historical series, and my brother happened to say it could be crime. I jotted that down as well. When I decided to switch to crime, I already had the basic scenario for the first book and I knew my heroine's name and something about her. She evolved differently as I wrote, but the seeds of the character were there long ago.
It's worth harnessing anything that floats into your head. Ideas are never wasted.
And a Happy New Writing Year to you all!