Has she got easy access to a toilet, or toilet facilities? Just wondering if she is exhausted and feeling unwell, especially as you said she won't wash or dress either, if she's stalling going to the loo (or drinking less to put off going) if it's say upstairs or something like that, and she just wants to stay on the sofa. In case she might benefit from something like a commode.
I'm glad that the problem seem to have resolved itself, Darcey, but if it happens again, don't wait too long before seeking medical help. Two years ago I (madly!) waited 11 days without being able to pee before calling an ambulance. (I was doubled up in pain and hardly able to breathe because my lungs were so full of fluid). I ended up spending 5 nights in hospital while they tried to get my kidneys working again, as they'd almost totally packed up due to the strain that they'd been under. It's definitely not something that I'd want anyone else to have to go through!