I can't get a decent picture of it, but the bulge is more predominant on its left side, probably closer to its anal area than its front.

No "pine-coning", and the fish seems to be fine otherwise- swimming normally, eating normally, but it looks quite grotesque.

I'll try to get a proper photo later and append it to this post, but anyone have any idea what's going on?

Anything to look for, in terms of why the tumour developed in the first place?

It would give the same bulge on just one side but usually the fish would swim with more difficulty, listing to one side or staying either on the surface or on the bottom. The difference is this condition can be treated. That is why I mention it.

A partial list of suggestions that could help:
"Feed your fish a couple of peas. That's right, peas. Just get some frozen peas, thaw them, and feed them to your fish. A professor of fish medicine at N.C. State College of Veterinary Medicine has done this in several cases with very good results. He thinks that the peas somehow encourage destruction of the impaction. No hard scientific data yet, but it's worth a try.
Fast your fish for a couple of days. Withhold all food for three or four days, and sometimes this alone will break up the impaction and return things to normal. Most fish can go a week to ten days without food and be just fine.
Periodic aspiration of the swim bladder works very well. Basically, you stick a needle in the swim bladder and suck out some of the air. Not something to be entered into lightly, but does work well. This is not a cure, but a successful treatment. The head veterinarian at the Baltimore Aquarium prefers this method.
Partial pneumocystectomy. This is another word for surgical removal of part of the swim bladder. I mention this less as a practical option but more just to let people know that there are vets out there doing X-rays, surgery, general anesthesia, even cancer chemotherapy."

I've never had a fish with a tumor but I have had one with a swim bladder problem. It resolved itself with the peas and gentle massage at the edge and lower back of the swelling. this might not help your fish at all if it is a tumor but it couldn't hurt. Some people prefer to cook the peas for a few seconds and then pop out the inside of each pea. Throw away the skin of the peas. It is tedious but my fish love them.

Sleepless, I think I'll try the peas; couldn't hurt.