I just wrote a big long response to another post of a bunny who just came home and isn't eating as much, so I'll copy-paste that here.
"The stress of moving could have caused this, combined with the change in pellet. Ideally, you should change the pellet over a week or two so as not to upset the stomach.
By now, stress over a new place would have subsided, so I'm guessing it's the pellet change. I assume he's been seen recently by a good vet (at the shelter).
The new food may be giving him gas or slowdown. I suspect gas because it's common when switching pellets and having stress, and he doesn't have a total stoppage of poo. Try giving him some simethicone--I use Mylicon, made for babies, in a liquid solution. I give 1mL/hr for the first 3 hours, and do gentle tummy massages, and offer a warm rice sock. This is just an old sock filled with uncooked rice put in the microwave for 1-2min. The heat helps with gas.
Another thing to give him is Bene-Bac. It's a probiotic and I give it to my guys whenever their tummies are a bit off--funny poos, lots of uneaten cecals, hairy poos, gas, etc. I prefer to get the big old syringe with each gram marked on the plunger that's made for dogs and cats. There are also small tubes of it made for dogs and cats that are fine. Some bunnies don't like the gel type, so some stores have it in powder form in the small animal section. It's best to get the dog/cat or small animal one, not the avian/reptile one in my opinion, but if all you can get is avian/reptile the ingredients are pretty similar. There's a similar product at farm supply stores called ProBios, which has the same ingredients as the mammal Bene-Bac and is in a gel that is made for horses.
Keep an eye on his input/output, and if the gas meds and Bene-Bac don't turn him around in 24 hours, he should see a vet. The fact that he's peeing less is worrisome, because that may mean he's drinking less too. The most important component of a rabbit's GI tract seems to be water, strangely enough. If he's not drinking a lot, try offering water in a bowl and a bottle, as he may prefer one over the other. You can also spike in a little apple or cranberry juice (not too much--I'd say dilute it 1:4 at least--the sugar can slow down the GI a little) or add a drop of vanilla flavoring to make his water tastier. Another option is to offer him unsweetened pedialyte. If he doesn't improve by vet time, he'll probably need sub-cutaneous fluids.
Another thing that some members swear by is fresh pineapple juice. You have to get a fresh pineapple and squeeze it, or offer him chunks of it. It has enzymes that supposedly help with digestion, and it contains water too. Again, though, there's a lot of sugar in it, so don't go overboard with it. The members who use it think that the enzyme and water in it outweigh the risks of adding sugar to an already slow GI tract. I prefer to give it as chunks because they have a bit more fiber.
And yet, another option is canned pumpkin. I put a little on a plate and it's tasty wet fiber that they seem to love. Make sure it doesn't have pie spices in it though.
Now that you've got a long shopping list, I'll end my novel! Good luck! "
Bene-Bac is good--give her a gram a day until she eats more. I hope you didn't do a rapid pellet change with her? I agree that water is a bigger deal. It sounds to me like maybe Kate has a tiny bit of gas or is in the early stages of slowdown, so offering her some of the things I mentioned (not forcing them on her--the stress would be counter-productive) along with Bene-Bac could help. I wouldn't do a simethicone treatment until you've given at least one dose of Bene-Bac without improvement.
