If a rabbit doesnt eat much, it doesn’t drink much either, as a rule.
Did the vet check her inner teeth (molars)? Rabbit teeth grow continuously and unless the teeth are worn down right by chewing hay, sharp spurs can develop on their teeth, and these spur can cause pain when the rabbit eats. You can’t see the molars yourself due to the anatomy of the rabbit’s mouth, so a vet needs to check them with an otoscope. Even small spurs can cause a bun to go off its food. Molar spurs can be trimmed away under full anesthesia, and then the bun goes back to eating like normal again after a couple of days.
Your bun might seem happier now diue to the pain medication. It’s in any case a good sign and makes it even more likely that this is a molar iassue and not something really bad. Do you support feed with Critical Care? You should do that to ensure that your bun doesn’t go with an empty tummy.