Yeah I thought this was a weird one! There is a small male Endkers guppy that has been persistently going at her, and I'm wondering if he has damaged her or something. Her belly is a little distended. I thought that she could be pregnant but now I'm not so sure. The tank is heavily planted, and parameters are good.
I would move it to a 10g and dump a TBSP per 5 gallons of rock sea salt and possibly a little epsom salt and crank up the heat, dont feed and see if it makes it. If you have methylene blue on hand why not put that in there as well, and an airstone.
Badly bloated, could be bacterial infection (just over abundance of canadia in gut) and needs clean fresh tank and heaps of salt to get it to release fluids (osmosis)