Those rules governing Twitter, Facebook, blogs, etc. too do they? It's only suspicious because you say it's suspicious.
As far as I can see, a 95 year old woman had a busy few weeks of engagements, she felt fatigued, doctors told her to rest and did some precautionary checks in hospital along with a night's observation. She went home, continued to rest, for longer than initially planned, undertook light virtual duties, spent a few days at her country retreat, came back and has now strained her back in some way.
My assumption is this was at least a few days ago (and didn't need commenting on as she was already "resting") and the hope was it would sufficiently settle down in time for Sunday's service. Clearly that didn't happen, so they've regretfully have had to say she can't attend. The fact she hasn't cancelled her attendance at the Cenotaph until the last possible moment, to me, isn't anything sinister, it just shows how determined she was to be present if at all possible.