What could be wrong with this kitten?

We take care of these stray cats around my house and one of them just had a litter of 5 kittens, and right now they will be six weeks old on Sunday. One of them isn’t doing so well, she’s the smallest of the litter and the skinniest, and her mother no longer wants to nurse. We introduced the kittens to solid foods, but the ill kitten refuses to eat and only wants to nurse from her mother. Perhaps her mother is rejecting her because she senses that something is wrong with her, I don’t really know. Anyway, the kitten’s private area is enlarged and swollen and she hasn’t been using the bathroom well so it’s been kind of dripping. Flies keep pestering her and we tried washing out the larvae they lay in her crouch area but there’s still tons more of them. We plan on getting her a bottle from the local grocery store to nurse her back to health. Do you have any idea at all what could be wrong with her?? And before you all recommend a vet, we simply can’t afford it. I am already paying off a bill from another kitten we had treated a few weeks back, so veterinary care isn’t an option. All of the local shelters and rescues are filled to capacity. We are basically her only hope, and at this point, we just want to make her comfortable and ensure she doesn’t die alone. Ideas on home remedies that would help would be greatly appreciated.