Hi and welcome to the forum
If the fish is still eating and swimming around normally, then it doesn't have a bacterial or protozoan infection. The fish could be constipated if it isn't getting any or enough plant matter in its diet.
What are you feeding the fish?
Has the fish done any poop over the last couple of days?
The fish might have a hernia or intestinal worms. There is nothing you can do about a hernia and intestinal worms don't normally cause the fish to develop a large swelling over night.
Because the fish is still eating, I would feed it some aquarium plants like Duckweed or Ambulia, or some partially cooked peas, pumpkin, zucchini or cucumber. All the fish can eat this and goldfish should have some plant matter in their diet each day.
See how the fish goes after eating some plant matter and monitor its poop. If the fish is eating but not pooping, there is a problem.
If the fish is eating plant matter the poop should be green.
If the fish has worms or an internal infection, the poop will often be white and stringy.
-----------------------
The Kanaplex is a strong anti-biotic that will wipe out filter bacteria and if used incorrectly can lead to drug resistant bacteria. Anti-biotics should only be used as a last resort to treat bacterial infections.
You should monitor the water quality for ammonia and nitrite over the next few weeks in case the filters have been affected by the medication.