I think you need to remember that in 1913 Alexandra didn't necessarily think she was at the end of her life. She had just entered her forties, and although life expectancy wasn't what it now is, unless informed of a terminal disease she still would not have considered herself "near the end of her life," nor would anyone else except, possibly, peasants for whom life expectancy was more limited. Many people, and eventually Alexandra herself, would comment that she seemed older looking, but this was due to the emotional and physical toll of her anxiety regarding Alexei and the stresses of the Russian political situation rather than numerical age.
That being said, I've also noticed Alexandra's nervous head bobbing in various clips, most especially the one from 1913. Consider, though, that Alexei was being carried at that time, and that the Romanovs were aware the event was being recorded on film and would be included in newsreels throughout the world. Alexandra didn't enjoy public spectacle to begin with, so why would she enjoy this particular event, knowing that her son and the heir to Russia's throne would be seen at such a disadvantage?
In addition to Alexandra's shyness and discomfort in public situations, it should also be noted that at this point in her life she might very well have been perimenopausal. Whatever our gender, and whether we are young, middle-aged, or older, I think most of us will concede that hormones and their fluctuations greatly inform our day-to-day moods and behaviors.