She sounds a bit like Lupin, he will scavange for an old dried up bit of cat food and turn his nose up at most other food.
Have you seen the vets since the op? There could be a reason that needs checking out.
Also what is the eather like, ours is very warm for the time of year and that could also be a reason.
Have you given her biscuits cos if mouth is painful they can just swallow them