Friend or foe?

I started writing something new yesterday. It wasn't what I intended to write yesterday. It was the start of a novel I was excited about writing, oh, about three years ago. What I wrote (1,000 word opening scene) wasn't in the style I would have chosen, three years ago. It was better. It might even be pretty good. Am I excited about it, however? No. Instead I am vaguely anxious about continuing with it, even opening the word document and looking at what I wrote yesterday. I feel as if my equilibirum has been unsettled. Threatened.

This isn't what I planned to write, when I was able to return to writing full-time. I had a plan, for goodness sake! I had notes - reams and reams of notes - character studies and character arcs. I knew where I was headed with it. This new thing? The cuckoo in my writer's nest? (Or is it a stork?) I have next to nothing. A one-page synopsis I wrote three years ago, to structure the story in my mind. No character studies. No plot, as such. No notes!! Just this threatening... itch. This idea that I could write this and it could be good, better than what I had planned.

Trouble with an itch? You scratch it, it might go away. Or flare up into something horrid.

Shouldn't I be wildly excited about writing something new? Isn't that a vital ingredient? Or, at least, hug-myself-in-secret excited?

I do feel just like a mother bird, who returns to her nest to hatch her egg and finds someone else's egg there instead. Should I settle and see what hatches? That's what happens in nature, yes?

Has anyone else ever experienced this sense of feeling threatened by what they're writing? Is it a danger sign? Should I step away, or hang around for what happens next?

Added: Usually at this time on a sunny morning, I am writing to the sound of seagulls outside. This morning, it's wood pigeons, and blackbirds.