How much has it ballooned up in the last few days?
When a fish looks like that and it gets fat over a day or two, it's usually caused by an internal bacterial infection and there's no cure. However, when they get an internal bacterial infection, they stop eating and do a stringy white poop. The fact the fish is still eating would suggest something else but it looks rather unpleasant.
Other reasons they get fat include eggs but they get fat over a period of months not days and they don't normally get that fat.
A tumour or cyst. Tumours take months to grow. Cysts can grow over a week or two and it might be a cyst. Fish will eat normally with a cyst or tumour and continue until they get too big and the cyst or tumour impedes feeding or breathing, then they die.
You can monitor it over the next few days but if it doesn't improve, I would euthanise it.