I'm worn out.
I think I accidentally sniffed up some dust (or it may have been catty-biscuit crumbs), and have had the catty-choos nearly all day.
I have tried nose-nudging things to see if that will help, but it didn't work.
In the end I gave up and had a nap. I have woken up now (see below), and the problem seems to have sorted itself, but I am still tired from all of the earlier catty-choos :(