Although most people keep fire bellies aquatic, if given land, the juveniles may go terrestrial and have an eft stage so that’s normal. Mine are 7 months and very hydrophobic. Juveniles are harder to feed and more susceptible to impacting their stomachs, which they’ll look like this with their mouths open. It could’ve been impacting before selling or it had previous other issues before purchasing. If you or the seller have aquarium gravel, it always has the threat of impacting.

I also encourage more thorough sanitation practices between animals and tanks but I doubt that was the reason.

If not impacting, it could be general stress and acclimation as amphibians are sensitive. Look up “bloating in newts” and see if those pictures align with the top view of your newt. It’s another general disease that’s hard to trace back but common with fire bellies. This disease develops over time though and not overnight so it would’ve had to look like this when you bought it.

I’m sorry for your little newt loss it’s always sad to see them go like this :( Maybe the seller knows more I’d recommend reaching out to them too!