I've Broken My Short Story Bone

Between working on In the Memory House and revising Borrowed Saints (which will have a shiny new cover and new title, coming soon), I've broken my short story bone.

I started a piece for Rainstorm Press's Mutation Nation anthology, but after two-thousand words have realized I'm writing a novella. Huh?

Waaaaay too much back story. I definately started in the wrong place.  This has never happened to me, and I'm struggling a bit with making it work.

Anyone else have similar issues when switching between longer and shorter writing?