Looking at the way the Queen has been walking / standing in the last few appearances she reminds me of my Gran just before she had a hip replacement.
The signs are familiar: needing a stick 'for comfort'; no longer riding (sitting legs akimbo when you have an arthritic hip is agony); being told to rest - again, with an arthritic hip, standing around can cause severe pain which can come on unexpectedly and be completely debilitating.
My personal feeling is that, yet again, rumours are being stoked by the press and she is in fact suffering from arthritis, a condition often associated with old age but which can be remedied - hence preliminary investigations in hospital.
For what it's worth, that's my two pennorth.