Another update:
The vet phoned today and told me that all the tests have come back as normal and so Tootsy doesn't have Cushings. She says it is probably just old age, but that she will be doing another set of blood tests in three months.
Tootsy has been a lot better this week. She has not been sleeping so much, or drinking quite as much, as last week.
I am relieved that she doesn't think that Toots has Cushings, but I can't help still feeling a little uneasy. At least she is brighter and doing more, so I am thankful for this - perhaps it was nothing serious?