Scottish koi club

I have had no problems with my pond or fish for a year or so know and water tests fine but a day ago one of them has started to lie on its side at the top of the water would i be right in thinking this is a swim bladder problem all the other fish are fine and are feeding well ,or could it be a sudden temp rise after the cold winter ?????? any advise would be welcome .

I am no expert A problem with the swimbladder could to blame, this isn't a disease itself but is actually a symptom of several causes, which include bacterial infections, dropsy, bloat and internal tumors or damage. Before we pin the blame on anything could you post some details on the water quality? Also how big is the tank/pond and does it have a filter?