Could be dropsy - a bacterial infection. It can be treated.

Dropsy in Koi Fish
http://www.koihealth.info/dropsy.html

does it look like a 'pine cone' from the tail end. if not it probably isn't dropsy. it could be a female that has eggs but has failed to spawn. I assume it is feeding ok.

Eggs are reabsorbed within a few days. She won't linger for months with a batch of unfertilized eggs. I think it's dropsy.

Not all fish are able to reabsorb their eggs. If the ovaries are immature they eggs may not be reabsorbed and the fish becomes egg bound. Egg binding causes females to bloat and may stay like that.
When the temperature rises again the ovaries may mature enough for the eggs to be released. The males bump into the females sides to induce egg laying.
A couple of years ago very frisky males managed to flip a female out of the pond. luckily one of my girls was in the garden and saw it and was able to pick her up and pop her back in the water. There were eggs all over the grass.

Producing eggs a few months ago during our Spring hot spell would also fit with the onset of symptoms.

Does the fish look as if the scales are sticking up giving it a pine cone appearance. if yes then it is dropsy and unlikely to recover.

Thanks for all the helpful comments.
No, the scales aren’t sticking out.
It’s looked like this for a couple of years!

in that case it is not dropsy. If its been like this for a couple of yrs now there is nothing you can do.

You said months.

Sorry Bathgate. I did. But then I asked my husband how long it was, and he said about two years. Time flies by when you’re enjoying yourself.....