She's looking very frail
I'd been thinking the complete opposite. When she's been filmed at engagements lately, she has genuinely seemed quite animated, cheerful (especially given her bereavement) and in good shape for someone her age. Even the use of a stick doesn't seem terrible for someone so elderly.
There is clearly more going on medically, it now seems (unless as pp have said, it's from having returned to doing too much too quickly). I can't see how it helps to make that public though. It shouldn't be a shock to anyone that a 95 year old isn't in the best of health. Details would only serve to start a kind of morbid symptom watch where the papers were effectively counting down to her death. Bit like Jade Goody when it became known how badly her terminal illness was now affecting her. I don't think Jade deserved that level of commentary on her final decline, and I don't think the Queen does either.
The Regency stuff seems way off the mark. The Queen has always been clear she will serve in her role until death. Charles can take up responsibilities without any formal declaration of a Regency, as he already has with some things. I hope the first we hear of anything more specific is that one day it will be announced that the Queen has died peacefully surrounded by loved ones and dogs. Till then, she's in her nineties and as far as I'm concerned she can take as much time off as she wants.
On a side note, I was really impressed by how lively David Attenborough was on the news earlier. He's 95 too and he can get up from a chair without grunting and groaning, which I find increasingly challenging even though I've got a number of decades on him.