It's quite likely because she hates being incontinent/ fears incontinence and going to the loo. So humiliating and distressing. So you just have to say to her you're going to take her there regularly. That way it's fine to eat and drink.
And the way we got round that, though never easy, was to try the following:
You did it for me when I couldn't, so now it's payback time when it's hard for you. [ But when you've eaten and got well, it will improve. And then you won't need me/ nurses anymore]
You'd do it for me, wouldn't you, if I needed you to? Well then.. repeat..[..]
It happens to everyone at some stage.. [...]
Let's go for a little walk....Oh good, we are just by the loo. Why don't we take the opportunity, as George V said..
You know how people have to be taught to walk and talk again? They also have to learn loos again. Well, you've made huge strides on the walking and talking, this is another thing you're going to conquer. Bet you do: bet a bottle of champagne on it,,,( made her giggle as she hated champagne and I don't).
(A bit like toddler training, with bits of humour. )
Reassure her, reassure her, reassure her.
A tiny bit of humour helps.
Reminding her how she was and is a stunner helps. Extra beautiful hand cream, getting her hair done, new bedwear, new scent...reading out her get well cards to her, even if she's confused as to who she is, the reassurance that she's a person is important.
If she was /is religious, get a chaplain to come along.. there'll be an RC/CoE/Protestant minister/rabbi almost certainly, if you're in a city there will be an imam or equivalent, and in Thomas's/Guys they have a Buddhist one as well. There's prob an atheist/agnostic/humanist counsellor too if I but knew.. they can be very comforting.
Hope helpful.