Maybe spring is a particularly effective time for writing. Last year I had some of the same experience as this-- one book led right into another. This particularly happens with secondary characters who excite my interest enough to find their story, which can be novella, short novel or long novel. Beginning a new project, at least if you don't have a contract, I don't think most authors know which length it will be. The story lasts as long as it takes to tell it.
If my writing work year was structured, as I feel is probably best, it would mean-- write a rough draft, edit the last one, edit the new one, etc. Then time for promoting the book that is due out, which means images for trailers and writing blurbs, not to mention articles wherever I have the opportunity to have the book seen. One for sure on writing-- if you can't get it seen, you can't get it sold.
Lately my writing process is not structured. It is unpredictable (last month at this time I had no idea I'd be writing what I am right now), fun (always neat to get to know new characters); but more driven than seems wise-- and not driven by the outside world but my inside one. Constantly writing new material tends to suck me into their world and out of mine. When I am not thrilled with my world, that can be good but on the other hand-- I wake up thinking-- if I had this happen, what would they do? And where my back and neck are saying-- are you insane?
Well, I might be. Some say all writers are a little insane because it is a calling where you are off in someone else's life a lot of the time. Now, your own life might be just fine, but you have to set aside its needs while you figure out those of these characters who never existed, never will, except in your imagination; and if you put that book out there-- the imagination of others.