I'm really sorry she has lost some more weight & has been grinding her teeth too too

Really hoping they can find something treatable for your lovely girl
Thank you so much, j&b.
We've just picked them up, and we're not really sure what exactly is going on yet, but there's something with her stomach

The vet said her bloods show high levels of inflammation, and that her stomach wall is much thicker than it should be in one spot. He said they don't have much experience with this in bunnies, though, so he wanted to have another vet look at the ultrasound, too, and discuss it with the other vets and look in some books and learn some more. I always appreciate it when a vet is honest about not knowing something rather than pretending to be all-knowing, so I'm okay with waiting while he looks into it. He should have a plan by Friday, he said. I asked what it could be, and he said it could be a tumour, but that he really can't say for certain at this point.
He also said that, unfortunately, he doesn't feel like it's safe to give her pain medication while we don't know what is wrong with her stomach, because even injectible pain relief will affect her stomach (the stomach acid, apparently), so that's not ideal at all, though I do understand it

I forgot to ask how this would explain all her drinking and peeing, but maybe drinking water soothes her stomach somehow, I'm not sure... I do find it amazing that she eats so much while her stomach is unwell. He said that she might not be absorbing enough/anything, and that that's why she's losing weight.
I very much hope it's something treatable, but I'm so worried about her. She's only been with Casper and me for just over two months, but I love her so much, and she and Casper are already becoming such good friends.