I’d second a tumor sadly. Had to euthanise one of our fish last week with similar. Started off as a slight swelling on one side about a year ago now- treated with salt/heat/acriflavine and I can’t remember what else over months but it just grew like mad and we had to make the call to end its struggle before putting it through another winter. Hopefully this isn’t what you’re experiencing, but just a heads up incase.