Rescued, spayed female. Her belly is HUGE, as if she's pregnant with TWO litters! Has been to the vet several times, but I'm unable to get a stool specimen (I have 5 other kitties, and she will not use another litter box 🙄). I also see blood in box, but don't know which cat it is, but is likely hers. These small black "gnat-like" flies hatch from the litter box. I Googled "worms that hatch" and got NOTHING. PLEASE help! I've never seen anything like this in my 50 years of having cats!
A possibility; these "gnat-like" flies could actually be fungus gnats or fruit flies. And Autumn is the usual time of year for them. If your litter is consistently damp and/or contains vegetable matter, such as corn or wood etc. both of which can attract these adult flies to the area to lay their eggs.
A huge belly (pregnant looking-and blood sighting) is a cause for concern. There are several reasons a swollen belly can occur - including: intestinal parasites, a heart condition, severe infection or hemorrhage of fluid buildup etc.
Keep in mind, that cats are masters at hiding thier illnesses, so please consider a trip for a second veterinarian opinion, as this condition will require a physical examination to determine an exact diagnosis and treatment plan right for her.
Please get every cat checked just to make certain that they are all healthy and to narrow which one has the bleeding... The flys will be attracted to the blood and urine in the litter box, but the gnats are bad this year and are everywhere , but please get them all checked and get a second opinion from a different vet on your new cat with the swollen belly.