It's not uncommon, in my experience. I've found it helpful to have a general overview of where I want to go, at least where I want to end up, but I never write complete plot outlines or go chapter by chapter and figure out what I want to have happen in each. Doing that takes half the fun out of writing, for me. I like not being entirely sure what's going to happen next, I like finding out as my characters do. I end up doing a lot more rewriting, but as long as I enjoy it, that's all that matters.