True diarrhea is rare in rabbits, but does occur in very young buns due to a parasite (coccidia) or a not yet fully developed gastro-intestinal microbiota that gets challenged by sth like stress or an abrupt change of food, and in buns that are on antibiotics. Antibiotics can kill off too many of the beneficial bacteria in the gut, leaving the field open for really nasty (but naturally occuring) bacteria to proliferate.
If she’s eating and drinking and has some normal poop, it’s soon Monday and you can call your vet.
There’s no universally tolerated veggie, and there are buns that just can’t have fresh vegs. Vegs that are often tolerated by rabbits with digestive issues are f ex mint (not in large quantities) and fennel. Buns can get allergic to certain foods like humans and dogs can. It could also be that her GI microbiota isn’t ready for veggies. To establish a healthy, sturdy GI flora, you generally give only hay for a longer period (plus pellets if that’s tolerated), about 3 months. Then you reintroduce greens like you would with a baby bunny.