I agree, constipation is normally an all round swelling not a lump in one place like this.
Do any other fish show any symptoms like this? With any of the fish not just this one, are there red bristles sticking out of the anus on the rare occasion the fish is motionless? Red bristles are camallanus worms, which go back inside the fish when it is swimming.
Given the apparent shape of the lump, which is always hard to see accurately in photos, I would say the most likely cause is a tumour, and I'm afraid there is no cure for that. If/when the fish is obviously suffering, that is the time to euthanise it, I'm afraid. If it comes to this point, ask about the best way to do it, as some of the methods you'll read about are cruel.