Could it be swim bladder disorder?

I know this is usually associated with floating but looking on google for sites about betta +illness found me that.

It says the betta may look off colour and that it may be lopsided, swim funny or just sink to the bottom.

Take a look at the site, it was the first one that came up. It says about doing a complete water change.

One other thing, I had a sick goldfish over the summer, he had swim bladder problems. I was told to feed him peas (which he loved) I was told to get a small frozen pee, defrost it in the microwave and give it to him. Also I was told not to feed the food on the top of the water, they ingest air as they eat it. I was told to either soak the flakes/pellets so they would sink, I don't soak them but push them down with my finger.

I do hope one of those sites can help you both further, I really hope that Kelsey hangs on and gets better, I have never seen a betta before but he is so handsome, the most beautiful fish I have ever seen.

Best wishes
Sarah