"Just a lump" isn't helping that much.
You could try posting a photo or something for instance (or link us to a pic), because it can be two things, a tumor, or carp pox.
Either way, there's nothing you can do about that. It may also be some other kind of infection, and then we'd need to see pics to make sure!!

You should, however, test the water right now. Check the pH, ammonia, nitrite and nitrate. And post the numerical results here.