has she been wormed ? might be bloated round the belly from worms it's quite common and no sign that you're a bad mummy. it's also easily fixed! but you will need to make sure she and her kittens are wormed. I believe Panacur paste would be the way to go but best to speak to your vets really and let them know what's going on. if it's worms no matter what food you feed she will still be bloated and still look under weight.

i know this sounds horrible but you can also speak to your vets about terminating the kittens if there are some and you don't think you will be able to find homes for them. depending on how far along she is if she is pregnant. you can also donate the kittens to your local charity should you decide to let her continue on to term.