Private Eye has a Palace Flunkey column - no idea if the "informant" is an actual employee, they don't disclose serious secrets, just drop hints and snide comments.
But the recent edition said Kate's surgery was expected to be minor and the plan had been in and out of hospital within the week, timed for last week of the holidays so no one would notice anything. Then it turned out to be more serious.
This sounds entirely possible to me.
Of course the "more serious" could be something like the surgeon accidentally snipped a ureter or something. A fairly common complication, we get a few coming through my department, often women having surgery for gynae reasons at the local private hospital....
It's not necessarily due to a crap surgeon, they can be difficult to identify. A recent case the surgeon had written "ureters identified and protected" in the operation notes yet somehow one was severed.
It would take a little while sorting out something like that so she's fit to return to a public role, looking fabulous and with a smile slapped in her face. No phased return to work for her. So it's entirely right that they gave a generous period of time for her to fully recover her health and stamina and hopefully she will return after Easter looking none the worse for whatever has been the problem.