I have no idea how to find the spear. I've never made one for just that reason. Throwing away my weapon in the middle of combat seems like a bad idea to me.

As for getting encumbered, that really shouldn't happen, but it seemed like that also happened to me recently, although not in combat. I wonder if it's a new bug?

FWIW, one of my first deaths was running away from what I think was my first encounter with some greydwarf, and picking up a pile of wood (on purpose) that encumbered me. It didn't take long after that...