Also, I’m sure this has already been commented on but… it all just makes sense to me. She looked absolutely lovely and vibrant, at a good weight, not tired, etc, at the Trooping of the Colour in June. By September, of course, still beautiful, but looking less like herself. And by December at her last commitment, of course still beautiful, but it was obvious she’d lost weight she wasn’t trying to lose, and had large circles under her eyes (she looked the same at the church on Christmas Day) - it looked to me like she could easily have been in a lot of genuine pain and putting off a “big surgery” until after Christmas for the children. There are so many things that can happen to women - almost all the same things that can happen to men, PLUS reproductive issues of all sorts, endometriosis and serious complications from it, complications years after births or c-sections. Whatever it is, it seems it was quite serious, and I do hope when she returns, she will be pain-free.
And like many others, it may be that she wants privacy while she is vulnerable and will share about the situation in her own time when she is recovered, and possibly help raise awareness for whatever it was. Who knows? It’s a different royal family than even 10 years ago.