i had guppies like that before. A few years ago. I had tried every internal parasite and de-wormer on the market, metronidazole, praziquentel, eventually fenbendazole. They prolapesed a little with the fenbendazole, and there were never any signs of worms on their vents.
Got my entire collection sick from some skinny looking "wild guppies" some shady kid on aquabid sold me. Never again will i go that route.
Could not ever figure out what it was, skinny and stringy poo, not eating, red dots and sores and tail rot. New guppies put in the tank would within a week baloon up and get bloated like this, then whirl and die.
I eventually was told by someone on here that it was probably TB, and i had to euthanize the entire tank and stearilize everything. I even took a sick fish to the vet, and there were no internal parasites or worms. I didnt know at the time much about TB, but should have had the vet look for "nodules" or whatever forms on the organs during a TB infection.
Not saying i know for sure what this is, because surely other bacteria and internal parasites can cause bloated and stringy poo, but just saying that was my experience.