My youth, it feels like moving!
That's a weird face expression.
is the expression of youth.
Now I feel old....
She just seems really excited about something.
In the story she starts out as a bored and restless highschool girl unhappy with the idea that her life is so humdrum. At the point of the story where this picture is (could actually be a screen shot I'm not sure) she and her friends are being chased around Shinjuku by some older women. (don't want to post a spoiler so I won't say why). Anyway while this is happening she realizes that she actually likes being in trouble. Hence the caption and expression.otaku_emmy said:
She just seems really excited about something.