Most likely, the cutout relay points in the regulator stuck the last time you turned the engine off, allowing the battery to discharge itself through the generator armature. This usually trashes both the generator and the regulator, and often melts the wiring, or at least the insulation, all the way from the battery cable connection at the starter, through the internal wiring harness to the ammeter, and from there to the regulator and to the armature terminal of the generator! If that didn't happen, consider yourself lucky! I've seen entire vehicles go up in smoke from a stuck cutout relay!
Be sure to change the regulator and inspect all the wire harness connections before putting your truck back in service!
Jerry
