First could be an abscess starting or scar from improper injection. Second could be scarring from a heat source, prey bite, or really bad shed.

He looks a little plump have you been able to weigh him? Also where did you get him from and do you have a qualified exotic vet nearby? I would highly recommend getting a fecal float done to rule out parasites. Even captive bred are known to harbor them from the prevalence of breeding stock swapping going on. It pays in the long run to have a check up done to make sure they are healthy. These wonderful snakes can get lethargic rather easy and don't show outward symptoms of issues so it is often better to be proactive rather than reactive.

The lump can be check for possibility of tumor or abscess infection and if infection can be taken care of before it gets bad.

I am assuming maybe 2+ years old and with the grounding make sure you are keeping basking temps proper and that you are observing night time activity to make sure he is not lethargic from weight issues or illness.

In short, would benefit you to get a vet checkup.