pat: If the nub is poking out you should go to an orthropedic clinic and get it checked out asap. I hurt myself a similar way 4 months ago, at first the doctors belived it to be a sprain because there was no fracture shown. But the navicular if fractured tends not to show up on xrays. In my case my injury turned out to be this. Link
I read about navicular problems on a sports injury site while trying to figure out if I had more than a sprain, and I do so I am glad I have a correct diagnosis and am being trated for it. (In a cast for a few weeks)