Does the cat wear a flea collar? Or any other kind of flea repellent. I once had a cat that was allergic to her flea collar.
Has a member of the family put on perfume, after-shave, changed shampoo?
Does the cat go out, because if so, you don't really know what he/ she is in contact with outside.
Cleaning agents can cause cats to sneeze, so perhaps it is your pre-Christmas cleaning that is the trouble.
Turning on or up the heating could cause it too, as the air will be drier.
I don't know whether our cat was bothered, but a month ago, my eyes started itching madly. The problem disappeared completely, when I opened every window in the house and left them open for twenty minutes. Worth trying, if you haven't already done so. It might just work.