This could be caused by a number of things, such as a swim bladder problem, constipation, or an internal infection.
✔ Make sure that the water quality is pristine (test for ammonia and nitrite using King British 6 in 1 Water Test Strips),
and the dissolved oxygen is high (by ensuring good aeration).
✔ Stop feeding for a day or two, to allow the gut to return to normal
✔ Adding a salt solution, such as King British Revitaliser Tonic, can help the fish to recover and regain the correct
balance of fluids in the body
N.B. Internal infections are notoriously difficult to treat without the use of antibiotics.