I will refute your vet and say that even if your rabbit is eating, she could still be in pain. She may eat at times she’s not in pain if it comes in bouts, but eating does not rule-out pain or discomfort. Wick still eats when he has dental issues and gas pains, so it’s not a good marker. You need to look at the whole context, which in your case includes loss of litter habits, excess water drinking, and a lump near her ovaries.
Was she spayed? I know many unspayed female rabbits develop cancer around there.
Her excessive drinking could also be a soother for any teeth irritation she has. While I do not know your rabbit’s situation dental-wise, your vet can see if her teeth is a current issue if he/she uses their otoscope to look in the mouth and if there’s excess saliva in there, it means your rabbit is producing more to lubricate and lessen the discomfort (which is also the reason why she may be drinking more water).
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.