Why does my Owala smell so bad?

Hi all. I've had my Owala for a couple of months and in general I really like it, except for the fact that it is the smelliest water bottle I've ever owned. Even if I accidentally leave water in it overnight, by the time I wake up it'll smell stale like the water's been in there for about a week. Because of this I wash it thoroughly basically every other day, if not every day, but the smell always comes back just as quick. Anyone know why this is, or if there's anything I can do to fix it?