How long has the lump been there? If its been more than a couple of hours I would take it to a vet.. There was a person who came here just a few weeks ago with a snake that only half swallowed its prey and it got stuck in its throat. It died a few days later.
Perhaps the vet can induce a regurgitation? I'm not really sure what they would do in this situation. Are you sure the snake swallowed a big chunk of substrate?