There’s something wrong with this young Platy.

[MysticFlwr]

I think it’s far too small to have swallowed a rock and too young to be pregnant. I don’t know what to do

 

[MysticFlwr]

Another photo for reference:

 

[Colin_T]

Some sort of internal growth, either a tumour or cyst. There's no cure for either and you euthanise when the fish stops eating or can't swim properly.

Tumours normally take a few months to develop into something like that. Cysts can take a few days to weeks.