It could also be swim bladder disease. Check pictures on google for both diseases (dropsy & SBD) and compare it to your fish. Like gookaluda said, it sounds like dropsy. That is highly contagious and it's due to bad water quality or bad conditions.

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GUPPY STRAINS I'M BREEDING:

Blue Grass
Blue Grass Swallow
Metal Blue Grass Swallow
Red Grass
Red Grass Swallow
Neon Blue Giessen/Ribbon
Japan Blue Blue Grass
Japan Blue Wild
Neon Blue

Strains I developed in my fish room:

HB Yellow (own line)
Platinum Pink Blue
HB Blue Leopard
HB Leopard Multi
HB Red Crowntail