The plumbing could be galvanized. The nipple could have rusted out ( this is the piece of pipe that comes from the elbow in the wall out the wall that the tub spout connects to. Depending on what is in the wall, and its condition, it could be easy or nasty to repair. You'll need to take some things apart in order to find out what really is the problem. Do you have access to the plumbing from say a closet so you don't have to tear up the wall in the shower?