Internal.
Many fish have parasites but when there is a stress of some kind the fish resistance could be lower and parasites get to be a bigger problem.
I had a wild Swordtail fry/baby and it grew up and then got to looking like that with the same hollow stomach issue. Did 2 rounds of medicated flake food one then the other about 3 weeks later and fish recovered and looked thin but normal again. It did not live as long as it should have but at least a year after the illness.
There weren't any other fish that looked like that nor it's parents in the tank where it was born.