My hot water started leaking from the tub spout, what does this mean?

I woke up to my hot water running fast into the tub, from the tub's spout (not the faucet handles) last night about 1am. It's been below freezing here and the hot water side of the tub faucet is close to an exterior wall of the house. I thought someone must have left the water running full blast, (kids??) so I shut it off only to discover that the faucet had a tiny one drop drip. I didn't think much about it, feeling like it was good to let it drip anyway, because of the cold weather. Then, this morning, my son took a shower and after turning off the faucets, the hot water is now basically still running, not full blast, but it's a lot more than a drip, (more like the faucet is actually turned on) though the handle is shut off. It can't be tightened any more now at all so it is basically running out all our hot water. I don't know what to do about this. Does anyone know what this means and how I can fix this? I can't afford a plumber, so I have to find a way to fix this myself. Your help is very much appreciated.