Sorry to hear about your mum, hope shes feeling a bit better now.
Just to let you know that I (a few times!) left my packs at home when I shoulf have had them at work, mainly because I am the most unorganised person in a morning ever! In my case it didnt ever cause a gain in weight, same 4lbs a week as per normal.
Water may retain for a day or so, as your body may hold on to it if its a bit low, but that is not true weight gain.
You are doing the right thing having a shake now, and try and have another 1 before bed, missing 1, for 1 day only will be ok.
With regards to water, by all means up your intake, but dont drown yourself, I have tried this before, and not only will you be up all night weeing, but you can also give yourself leg cramps if you down a large amount before bed.
In these cases, its a circumstance that is beyond your control. Dont fret, and go for it tomorrow x