Two big possibilities that come to my mind are either swim bladder problems, or something called whirling disease. Swim bladder problems are much much much more common, but also have many causes. If the fish looks bloated, try to feed it a pea, as it may be constipated. At the same time, a round of antibiotics can also be helpful. Even though it may not be bacterial, my theory is it is better to treat fish sooner with antibiotics, as the more advanced a disease, the harder it is to kill.
Constipation and Bacterial infections are the two easiest to cure causes of swim bladder problems. Unfortunately, whirling disease is basically impossible to cure. If possible, i would isolate the fish.
I know it is hard to watch. I once had a yellow lab do something similar, where he literally swam in corkscrews nonstop for a week. Even at night with the lights out. Unfortunately, no antibacterial meds helped, and I euthanized him.
Good luck!