Ouch.... ur guppies have a columnaris worm infestation. This is an internal parasite that sucks it's food off the guts of the fish leaving the fish malnourished (ie bent spines etc.). It appears fat because the belly region is full of these worms! Often this is a big problem with fish in LFS as many are left untreated.

The treatment I have been successful with is a 3 day dose of Flagyl. Active ingredient is Metronidazole. Other products that contain this active ingredient is Octozin by Waterlife.

Treat the fish ASAP.