Liz Buchanan BVSc MRCVS
Hello! So let me clarify: your cat has soft poops (yellow for a while, now returning to brown), is underweight and drinking a lot, and has had some antibiotics (but not from a vet? I trust you are not in the UK, where we are based- that would be illegal here). They are also eating a lot of fibre.
It's worth remembering that cats are carnivores, not really designed to eat or digest substantial amounts of fibre. Either this, or the antibiotics (I am going to assume for the moment that these were species and dose - appropriate), could actually cause runny faeces, sore bottom and large tummy in itself. However, there is a long list of other problems that could also be involved here.
Furthermore, there is only so much fluid that a kitten can lose before they become dehydrated and seriously ill. The reason for their extreme water-drinking may be to do with dehydration, or they may be managing to keep up with the fluid-loss in the faeces by drinking so much. It is hard to tell from here. Please see a vet as soon as possible to get them checked and, if nothing else, to advise you a good, kitten-appropriate food for your cat.